关于组合的回溯/递归方法的说明。关于递归调用中的行为的问题

Question

nums splice 和 temp pop 操作在第一次返回条件后运行两次。为什么？我已经在 console.log 中输出了所有内容以便理解，但无法得到以下打印两次的原因：：

::new after rec call:: i::  0  temp::  [ 1, 2, 3 ]  nums::  []
after rec call:: temp::  [ 1, 2 ]  nums::  [ 3 ]
outside for loop::  [ 1, 2 ]  nums::  [ 3 ]
::new after rec call:: i::  0  temp::  [ 1, 2 ]  nums::  [ 3 ]
after rec call:: temp::  [ 1 ]  nums::  [ 2, 3 ]

据我了解，由于返回条件，它可以在返回后执行一次操作，因为流程将返回到调用函数的行。 对此的解释将不胜感激。

const combination = (nums) => {
  let temp = []
  let result = []

  function backtracking(temp, nums) {
    console.log("temp:: ", temp, " nums:: ", nums);
    if (nums.length === 0) {
      result.push([...temp])
      console.log("::::: result.push::  temp:: ", temp, " nums:: ", nums);
      return
    }

    for (let i = 0; i < nums.length; i++) {
      console.log("::temp.push::  i:: ", i, " temp:: ", temp, " nums:: ", nums);
      temp.push(nums[i]);
      nums.splice(i, 1);
      console.log("before rec call:: i:: ", i, " temp:: ", temp, " nums:: ", nums);
      backtracking(temp, nums);
      console.log("::new after rec call:: i:: ", i, " temp:: ", temp, " nums:: ", nums);
      nums.splice(i, 0, temp.pop());
      console.log("after rec call:: temp:: ", temp, " nums:: ", nums);
    }
    console.log("outside for loop:: ", temp, " nums:: ", nums);
  }
  console.log("before calling bakctracking:: ", temp, " nums:: ", nums);
  backtracking(temp, nums)
  console.log(" result:: ", result);
}
combination([1, 2, 3]);

以下是所有控制台日志的输出::

before calling bakctracking::  []  nums::  [ 1, 2, 3 ]
temp::  []  nums::  [ 1, 2, 3 ]
::temp.push::  i::  0  temp::  []  nums::  [ 1, 2, 3 ]
before rec call:: i::  0  temp::  [ 1 ]  nums::  [ 2, 3 ]
temp::  [ 1 ]  nums::  [ 2, 3 ]
::temp.push::  i::  0  temp::  [ 1 ]  nums::  [ 2, 3 ]
before rec call:: i::  0  temp::  [ 1, 2 ]  nums::  [ 3 ]
temp::  [ 1, 2 ]  nums::  [ 3 ]
::temp.push::  i::  0  temp::  [ 1, 2 ]  nums::  [ 3 ]
before rec call:: i::  0  temp::  [ 1, 2, 3 ]  nums::  []
temp::  [ 1, 2, 3 ]  nums::  []
::::: result.push::  temp::  [ 1, 2, 3 ]  nums::  []
::new after rec call:: i::  0  temp::  [ 1, 2, 3 ]  nums::  []//from this log --
after rec call:: temp::  [ 1, 2 ]  nums::  [ 3 ]
outside for loop::  [ 1, 2 ]  nums::  [ 3 ]
::new after rec call:: i::  0  temp::  [ 1, 2 ]  nums::  [ 3 ]
after rec call:: temp::  [ 1 ]  nums::  [ 2, 3 ]              // --to this log
::temp.push::  i::  1  temp::  [ 1 ]  nums::  [ 2, 3 ]
before rec call:: i::  1  temp::  [ 1, 3 ]  nums::  [ 2 ]
temp::  [ 1, 3 ]  nums::  [ 2 ]
::temp.push::  i::  0  temp::  [ 1, 3 ]  nums::  [ 2 ]
before rec call:: i::  0  temp::  [ 1, 3, 2 ]  nums::  []
temp::  [ 1, 3, 2 ]  nums::  []
::::: result.push::  temp::  [ 1, 3, 2 ]  nums::  []
::new after rec call:: i::  0  temp::  [ 1, 3, 2 ]  nums::  []
after rec call:: temp::  [ 1, 3 ]  nums::  [ 2 ]
outside for loop::  [ 1, 3 ]  nums::  [ 2 ]
::new after rec call:: i::  1  temp::  [ 1, 3 ]  nums::  [ 2 ]
after rec call:: temp::  [ 1 ]  nums::  [ 2, 3 ]
outside for loop::  [ 1 ]  nums::  [ 2, 3 ]
::new after rec call:: i::  0  temp::  [ 1 ]  nums::  [ 2, 3 ]
after rec call:: temp::  []  nums::  [ 1, 2, 3 ]
::temp.push::  i::  1  temp::  []  nums::  [ 1, 2, 3 ]
before rec call:: i::  1  temp::  [ 2 ]  nums::  [ 1, 3 ]
temp::  [ 2 ]  nums::  [ 1, 3 ]
::temp.push::  i::  0  temp::  [ 2 ]  nums::  [ 1, 3 ]
before rec call:: i::  0  temp::  [ 2, 1 ]  nums::  [ 3 ]
temp::  [ 2, 1 ]  nums::  [ 3 ]
::temp.push::  i::  0  temp::  [ 2, 1 ]  nums::  [ 3 ]
before rec call:: i::  0  temp::  [ 2, 1, 3 ]  nums::  []
temp::  [ 2, 1, 3 ]  nums::  []
::::: result.push::  temp::  [ 2, 1, 3 ]  nums::  []
::new after rec call:: i::  0  temp::  [ 2, 1, 3 ]  nums::  []
after rec call:: temp::  [ 2, 1 ]  nums::  [ 3 ]
outside for loop::  [ 2, 1 ]  nums::  [ 3 ]
::new after rec call:: i::  0  temp::  [ 2, 1 ]  nums::  [ 3 ]
after rec call:: temp::  [ 2 ]  nums::  [ 1, 3 ]
::temp.push::  i::  1  temp::  [ 2 ]  nums::  [ 1, 3 ]
before rec call:: i::  1  temp::  [ 2, 3 ]  nums::  [ 1 ]
temp::  [ 2, 3 ]  nums::  [ 1 ]
::temp.push::  i::  0  temp::  [ 2, 3 ]  nums::  [ 1 ]
before rec call:: i::  0  temp::  [ 2, 3, 1 ]  nums::  []
temp::  [ 2, 3, 1 ]  nums::  []
::::: result.push::  temp::  [ 2, 3, 1 ]  nums::  []
::new after rec call:: i::  0  temp::  [ 2, 3, 1 ]  nums::  []
after rec call:: temp::  [ 2, 3 ]  nums::  [ 1 ]
outside for loop::  [ 2, 3 ]  nums::  [ 1 ]
::new after rec call:: i::  1  temp::  [ 2, 3 ]  nums::  [ 1 ]
after rec call:: temp::  [ 2 ]  nums::  [ 1, 3 ]
outside for loop::  [ 2 ]  nums::  [ 1, 3 ]
::new after rec call:: i::  1  temp::  [ 2 ]  nums::  [ 1, 3 ]
after rec call:: temp::  []  nums::  [ 1, 2, 3 ]
::temp.push::  i::  2  temp::  []  nums::  [ 1, 2, 3 ]
before rec call:: i::  2  temp::  [ 3 ]  nums::  [ 1, 2 ]
temp::  [ 3 ]  nums::  [ 1, 2 ]
::temp.push::  i::  0  temp::  [ 3 ]  nums::  [ 1, 2 ]
before rec call:: i::  0  temp::  [ 3, 1 ]  nums::  [ 2 ]
temp::  [ 3, 1 ]  nums::  [ 2 ]
::temp.push::  i::  0  temp::  [ 3, 1 ]  nums::  [ 2 ]
before rec call:: i::  0  temp::  [ 3, 1, 2 ]  nums::  []
temp::  [ 3, 1, 2 ]  nums::  []
::::: result.push::  temp::  [ 3, 1, 2 ]  nums::  []
::new after rec call:: i::  0  temp::  [ 3, 1, 2 ]  nums::  []
after rec call:: temp::  [ 3, 1 ]  nums::  [ 2 ]
outside for loop::  [ 3, 1 ]  nums::  [ 2 ]
::new after rec call:: i::  0  temp::  [ 3, 1 ]  nums::  [ 2 ]
after rec call:: temp::  [ 3 ]  nums::  [ 1, 2 ]
::temp.push::  i::  1  temp::  [ 3 ]  nums::  [ 1, 2 ]
before rec call:: i::  1  temp::  [ 3, 2 ]  nums::  [ 1 ]
temp::  [ 3, 2 ]  nums::  [ 1 ]
::temp.push::  i::  0  temp::  [ 3, 2 ]  nums::  [ 1 ]
before rec call:: i::  0  temp::  [ 3, 2, 1 ]  nums::  []
temp::  [ 3, 2, 1 ]  nums::  []
::::: result.push::  temp::  [ 3, 2, 1 ]  nums::  []
::new after rec call:: i::  0  temp::  [ 3, 2, 1 ]  nums::  []
after rec call:: temp::  [ 3, 2 ]  nums::  [ 1 ]
outside for loop::  [ 3, 2 ]  nums::  [ 1 ]
::new after rec call:: i::  1  temp::  [ 3, 2 ]  nums::  [ 1 ]
after rec call:: temp::  [ 3 ]  nums::  [ 1, 2 ]
outside for loop::  [ 3 ]  nums::  [ 1, 2 ]
::new after rec call:: i::  2  temp::  [ 3 ]  nums::  [ 1, 2 ]
after rec call:: temp::  []  nums::  [ 1, 2, 3 ]
outside for loop::  []  nums::  [ 1, 2, 3 ]
result::  [
[ 1, 2, 3 ],
[ 1, 3, 2 ],
[ 2, 1, 3 ],
[ 2, 3, 1 ],
[ 3, 1, 2 ],
[ 3, 2, 1 ]
]

Answer 1

这不是一个答案，而只是一种让您更轻松地查看您正在做的事情的更清晰日志、跟踪日志语句中的递归深度并简化记录的数据的方法。

这个输出是不是更清楚了？

before calling bakctracking - {"temp":[],"nums":[1,2,3]}
     entering backtracking - {"temp":[],"nums":[1,2,3],"depth":1}
     temp.push - {"i":0,"temp":[],"nums":[1,2,3]}
     before rec call - {"i":0,"temp":[1],"nums":[2,3]}
         entering backtracking - {"temp":[1],"nums":[2,3],"depth":2}
         temp.push - {"i":0,"temp":[1],"nums":[2,3]}
         before rec call - {"i":0,"temp":[1,2],"nums":[3]}
             entering backtracking - {"temp":[1,2],"nums":[3],"depth":3}
             temp.push - {"i":0,"temp":[1,2],"nums":[3]}
             before rec call - {"i":0,"temp":[1,2,3],"nums":[]}
                 entering backtracking - {"temp":[1,2,3],"nums":[],"depth":4}
                 result.push - {"temp":[1,2,3],"nums":[]}
             new after rec call - {"i":0,"temp":[1,2,3],"nums":[]}
             after rec call - {"temp":[1,2],"nums":[3]}
             outside for loop - {"temp":[1,2],"nums":[3]}
         new after rec call - {"i":0,"temp":[1,2],"nums":[3]}
         after rec call - {"temp":[1],"nums":[2,3]}
         temp.push - {"i":1,"temp":[1],"nums":[2,3]}
         before rec call - {"i":1,"temp":[1,3],"nums":[2]}
             entering backtracking - {"temp":[1,3],"nums":[2],"depth":3}
             temp.push - {"i":0,"temp":[1,3],"nums":[2]}
             before rec call - {"i":0,"temp":[1,3,2],"nums":[]}
                 entering backtracking - {"temp":[1,3,2],"nums":[],"depth":4}
                 result.push - {"temp":[1,3,2],"nums":[]}
             new after rec call - {"i":0,"temp":[1,3,2],"nums":[]}
             after rec call - {"temp":[1,3],"nums":[2]}
             outside for loop - {"temp":[1,3],"nums":[2]}
         new after rec call - {"i":1,"temp":[1,3],"nums":[2]}
         after rec call - {"temp":[1],"nums":[2,3]}
         outside for loop - {"temp":[1],"nums":[2,3]}
     new after rec call - {"i":0,"temp":[1],"nums":[2,3]}
     after rec call - {"temp":[],"nums":[1,2,3]}
     temp.push - {"i":1,"temp":[],"nums":[1,2,3]}
     before rec call - {"i":1,"temp":[2],"nums":[1,3]}
         entering backtracking - {"temp":[2],"nums":[1,3],"depth":2}
         temp.push - {"i":0,"temp":[2],"nums":[1,3]}
         before rec call - {"i":0,"temp":[2,1],"nums":[3]}
             entering backtracking - {"temp":[2,1],"nums":[3],"depth":3}
             temp.push - {"i":0,"temp":[2,1],"nums":[3]}
             before rec call - {"i":0,"temp":[2,1,3],"nums":[]}
                 entering backtracking - {"temp":[2,1,3],"nums":[],"depth":4}
                 result.push - {"temp":[2,1,3],"nums":[]}
             new after rec call - {"i":0,"temp":[2,1,3],"nums":[]}
             after rec call - {"temp":[2,1],"nums":[3]}
             outside for loop - {"temp":[2,1],"nums":[3]}
         new after rec call - {"i":0,"temp":[2,1],"nums":[3]}
         after rec call - {"temp":[2],"nums":[1,3]}
         temp.push - {"i":1,"temp":[2],"nums":[1,3]}
         before rec call - {"i":1,"temp":[2,3],"nums":[1]}
             entering backtracking - {"temp":[2,3],"nums":[1],"depth":3}
             temp.push - {"i":0,"temp":[2,3],"nums":[1]}
             before rec call - {"i":0,"temp":[2,3,1],"nums":[]}
                 entering backtracking - {"temp":[2,3,1],"nums":[],"depth":4}
                 result.push - {"temp":[2,3,1],"nums":[]}
             new after rec call - {"i":0,"temp":[2,3,1],"nums":[]}
             after rec call - {"temp":[2,3],"nums":[1]}
             outside for loop - {"temp":[2,3],"nums":[1]}
         new after rec call - {"i":1,"temp":[2,3],"nums":[1]}
         after rec call - {"temp":[2],"nums":[1,3]}
         outside for loop - {"temp":[2],"nums":[1,3]}
     new after rec call - {"i":1,"temp":[2],"nums":[1,3]}
     after rec call - {"temp":[],"nums":[1,2,3]}
     temp.push - {"i":2,"temp":[],"nums":[1,2,3]}
     before rec call - {"i":2,"temp":[3],"nums":[1,2]}
         entering backtracking - {"temp":[3],"nums":[1,2],"depth":2}
         temp.push - {"i":0,"temp":[3],"nums":[1,2]}
         before rec call - {"i":0,"temp":[3,1],"nums":[2]}
             entering backtracking - {"temp":[3,1],"nums":[2],"depth":3}
             temp.push - {"i":0,"temp":[3,1],"nums":[2]}
             before rec call - {"i":0,"temp":[3,1,2],"nums":[]}
                 entering backtracking - {"temp":[3,1,2],"nums":[],"depth":4}
                 result.push - {"temp":[3,1,2],"nums":[]}
             new after rec call - {"i":0,"temp":[3,1,2],"nums":[]}
             after rec call - {"temp":[3,1],"nums":[2]}
             outside for loop - {"temp":[3,1],"nums":[2]}
         new after rec call - {"i":0,"temp":[3,1],"nums":[2]}
         after rec call - {"temp":[3],"nums":[1,2]}
         temp.push - {"i":1,"temp":[3],"nums":[1,2]}
         before rec call - {"i":1,"temp":[3,2],"nums":[1]}
             entering backtracking - {"temp":[3,2],"nums":[1],"depth":3}
             temp.push - {"i":0,"temp":[3,2],"nums":[1]}
             before rec call - {"i":0,"temp":[3,2,1],"nums":[]}
                 entering backtracking - {"temp":[3,2,1],"nums":[],"depth":4}
                 result.push - {"temp":[3,2,1],"nums":[]}
             new after rec call - {"i":0,"temp":[3,2,1],"nums":[]}
             after rec call - {"temp":[3,2],"nums":[1]}
             outside for loop - {"temp":[3,2],"nums":[1]}
         new after rec call - {"i":1,"temp":[3,2],"nums":[1]}
         after rec call - {"temp":[3],"nums":[1,2]}
         outside for loop - {"temp":[3],"nums":[1,2]}
     new after rec call - {"i":2,"temp":[3],"nums":[1,2]}
     after rec call - {"temp":[],"nums":[1,2,3]}
     outside for loop - {"temp":[],"nums":[1,2,3]}
 result - [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

您可以通过扩展此 sn-p 来查看我对您的日志记录所做的细微调整：

const {log, display} = ((lines = []) => ({
  log: (depth) => (msg, obj) => lines .push (`${'    ' .repeat (depth)} ${msg} - ${JSON .stringify (obj)}`),
  display: () => console .log (lines .join('\n'))
}))() 

const combination = (nums) => {
  let temp = []
  let result = []

  function backtracking(temp, nums, depth) {
    log (depth) ("entering backtracking", {temp, nums, depth});
    if (nums.length === 0) {
      result.push([...temp])
      log (depth) ("result.push", {temp, nums});
      return
    }

    for (let i = 0; i < nums.length; i++) {
      log (depth) ("temp.push", {i, temp, nums});
      temp.push(nums[i]);
      nums.splice(i, 1);
      log (depth) ("before rec call", {i, temp, nums});
      backtracking(temp, nums, depth + 1);
      log (depth) ("new after rec call", {i, temp, nums});
      nums.splice(i, 0, temp.pop());
      log (depth) ("after rec call", {temp, nums});
    }
    log (depth) ("outside for loop", {temp, nums});
  }
  log (0) ("before calling bakctracking", {temp, nums});
  backtracking(temp, nums, 1)
  log (0) ("result", result);
}

combination ([1, 2, 3]);

display ()

.as-console-wrapper {max-height: 100% !important; top: 0}