C#如何使用堆栈优化我的代码？ （HackerRank“等栈”问题）

Question

我已经解决了这个问题，但是 11/30 测试用例因为“超出时间限制”而失败。我想知道是否有办法优化我的代码，以便解决方案通过所有测试用例。

问题的链接： https://www.hackerrank.com/challenges/equal-stacks/problem

我的方法： 正如问题的名称所暗示的那样，我想使用堆栈，即使方法中传递的参数是 List。 所以首先我通过以下方式将列表转换为堆栈：每个顶部元素表示堆栈的当前高度。因此，如果有高度为 1、5、10 的块，则堆栈中的元素将为 1、6、16。 我的算法是从堆栈中弹出最大元素，直到所有堆栈具有相同的高度。

这是我的代码：

public static int equalStacks(List<int> h1, List<int> h2, List<int> h3)
{
    var result = 0;
    if (IsEmpty(h1) || IsEmpty(h2) || IsEmpty(h3)) return result;
    
    var h1Stack = GetStack(h1);
    var h2Stack = GetStack(h2);
    var h3Stack = GetStack(h3);
    
    while (h1Stack.Any() && h2Stack.Any() && h3Stack.Any())
    {
        var h1Height = LocalPeek(h1Stack);
        var h2Height = LocalPeek(h2Stack);
        var h3Height = LocalPeek(h3Stack);
    
        if (h1Height == h2Height && h1Height == h3Height) return CountHeight(h1Stack);
        
        var max = Math.Max(h1Height, Math.Max(h2Height, h3Height));
        if (h1Height == max && h1Stack.Any()) h1Stack.Pop();
        if (h2Height == max && h2Stack.Any()) h2Stack.Pop();
        if (h3Height == max && h3Stack.Any()) h3Stack.Pop();
    }
    
    return 0;
}

static bool IsEmpty(List<int> stack) 
{
    return (!stack?.Any() ?? true);
}

static int LocalPeek(Stack<int> stack){
    if (stack.Any()) return stack.Peek();
    return 0;
}

static Stack<int> GetStack(List<int> numbers)
{
    var stack = new Stack<int>(numbers.Count);

    for (var i = numbers.Count - 1; i >= 0; --i)
    {
        stack.Push(numbers[i] + LocalPeek(stack));
    }

    return stack;
}

static int CountHeight(Stack<int> stack)
{
    return stack.Any() ? stack.Peek() : 0;
}

如果我们有长度列表：m，n，k 作为输入，n 是它们的最大值，那么我认为我的算法在 O(n) 中执行；老实说，我认为它没有任何问题。 如果我做错了什么，请告诉我，我该如何解决。

谢谢。

Answer 1

我还建议不要使用堆栈，这是一种非 linq 方法，每个列表（一次解析）上的 O(n)，而且它分配的数量不超过计算出的变量。

public static int EqualStacks(int[] h1, int[] h2, int[] h3)
{
   var a = new[] {h1, h2, h3};
   var i = new int[3];
   var c = new int[3];
   var last = 0;

   bool GetLowest(out int result)
   {
      result = -1;
      if (c[0] <= c[1] && c[0] <= c[2] && i[0] < a[0].Length-1) result = 0;
      else if (c[1] <= c[0] && c[1] <= c[2] && i[1] < a[1].Length - 1) result =  1;
      else if (i[2] < a[2].Length - 1) result = 2;
      else return false;
      return true;
   }

   while (true)
   {
      if (!GetLowest(out var index)) 
         return last;

      c[index] += a[index][i[index]];
      i[index]++;

      if (c[0] == c[1] && c[1] == c[2])
         last = c[0];

      // Console.WriteLine($"Current = {string.Join(", ", c)}, Index = {index}, Last = {last}");
   }

}

用法

var result = EqualStacks(
   new[] {1, 1, 1, 2, 3},
   new[] {2, 3, 4},
   new[] {1, 4, 1, 1});
Console.WriteLine(result);

结果

Current = 1, 0, 0, Index = 0, Last = 0
Current = 1, 2, 0, Index = 1, Last = 0
Current = 1, 2, 1, Index = 2, Last = 0
Current = 2, 2, 1, Index = 0, Last = 0
Current = 2, 2, 5, Index = 2, Last = 0
Current = 3, 2, 5, Index = 0, Last = 0
Current = 3, 5, 5, Index = 1, Last = 0
Current = 5, 5, 5, Index = 0, Last = 5
Current = 5, 5, 6, Index = 2, Last = 5
Result = 5, 5, 6, Last = 5
5

_{注意：虽然我的大脑说它有效，但我还没有彻底测试过。. 会出什么问题（不好意思地说）？ ¯\_(ツ)_/¯,}

您还可以内联GetLowest()，这样可以省去讨厌的堆栈调用和其他恶作剧，并且您可以摆脱数组或堆栈分配它们

[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static bool GetLowest(out int result, int[] c, int[] i, int[][] a)

基准测试

Method	N	Mean	Error	StdDev	Gen 0	Gen 1	Allocated
Mine	10	90.89 ns	0.581 ns	0.544 ns	0.0153	-	128 B
Yours	10	2,069.31 ns	13.881 ns	12.984 ns	0.0343	-	288 B
Dmitrys	10	304.75 ns	1.331 ns	1.245 ns	0.0143	-	120 B
Mine	1000	8,626.12 ns	108.481 ns	101.473 ns	0.0153	-	128 B
Yours	1000	43,196.78 ns	327.480 ns	290.302 ns	1.4038	-	12,168 B
Dmitrys	1000	20,563.90 ns	64.089 ns	53.517 ns	-	-	120 B
Mine	10000	202,904.36 ns	684.727 ns	571.778 ns	-	-	128 B
Yours	10000	402,362.80 ns	3,263.325 ns	3,052.517 ns	14.1602	1.4648	120,168 B
Dmitrys	10000	205,329.07 ns	1,014.000 ns	948.496 ns	-	-	120 B

代码

[SimpleJob(RuntimeMoniker.Net50)]

[MemoryDiagnoser]
public class WeirdStuff
{

   private int[] data1;
   private int[] data2;
   private int[] data3;
   private List<int> h1;
   private List<int> h2;
   private List<int> h3;
   [Params(10, 1000, 10000)] public int N;

   [GlobalSetup]
   public void Setup()
   {
      var r = new Random(42);
      data1 = Enumerable.Range(0, N).Select(x => r.Next(1, 5)).ToArray();
      data2 = Enumerable.Range(0, N).Select(x => r.Next(1, 5)).ToArray();
      data3 = Enumerable.Range(0, N).Select(x => r.Next(1, 5)).ToArray();
      h1 = new List<int>(data1);
      h2 = new List<int>(data2);
      h3 = new List<int>(data3);
   }

   [Benchmark]
   public int Mine()
   {
      var a = new[] {data1, data2, data3};
      var i = new int[3];
      var c = new int[3];
      var last = 0;


      while (true)
      {
         if (!GetLowest(out var index, c, i, a))
            return last;

         c[index] += a[index][i[index]];
         i[index]++;

         if (c[0] == c[1] && c[1] == c[2])
            last = c[0];

         //Console.WriteLine($"Current = {string.Join(", ", c)}, Index = {index}, Last = {last}");
      }
   }

   [MethodImpl(MethodImplOptions.AggressiveInlining)]
   private static bool GetLowest(out int result, int[] c, int[] i, int[][] a)
   {
      result = -1;
      if (c[0] <= c[1] && c[0] <= c[2] && i[0] < a[0].Length - 1) result = 0;
      else if (c[1] <= c[0] && c[1] <= c[2] && i[1] < a[1].Length - 1) result = 1;
      else if (i[2] < a[2].Length - 1) result = 2;
      else return false;
      return true;
   }

   [Benchmark]
   public int Yours()
   {
      var result = 0;
      if (IsEmpty(h1) || IsEmpty(h2) || IsEmpty(h3)) return result;

      var h1Stack = GetStack(h1);
      var h2Stack = GetStack(h2);
      var h3Stack = GetStack(h3);

      while (h1Stack.Any() && h2Stack.Any() && h3Stack.Any())
      {
         var h1Height = LocalPeek(h1Stack);
         var h2Height = LocalPeek(h2Stack);
         var h3Height = LocalPeek(h3Stack);

         if (h1Height == h2Height && h1Height == h3Height) return CountHeight(h1Stack);

         var max = Math.Max(h1Height, Math.Max(h2Height, h3Height));
         if (h1Height == max && h1Stack.Any()) h1Stack.Pop();
         if (h2Height == max && h2Stack.Any()) h2Stack.Pop();
         if (h3Height == max && h3Stack.Any()) h3Stack.Pop();
      }

      return 0;
   }

   bool IsEmpty(List<int> stack)
   {
      return (!stack?.Any() ?? true);
   }

   int LocalPeek(Stack<int> stack)
   {
      if (stack.Any()) return stack.Peek();
      return 0;
   }

   Stack<int> GetStack(List<int> numbers)
   {
      var stack = new Stack<int>(numbers.Count);

      for (var i = numbers.Count - 1; i >= 0; --i)
      {
         stack.Push(numbers[i] + LocalPeek(stack));
      }

      return stack;
   }

   static int CountHeight(Stack<int> stack)
   {
      return stack.Any() ? stack.Peek() : 0;
   }

   [Benchmark]
   public int Dmitrys()
   {
      if (h1.Count <= 0 || h2.Count <= 0 || h3.Count <= 0)
         return 0;

      int height1 = h1.Sum();
      int height2 = h2.Sum();
      int height3 = h3.Sum();

      int p1 = 0;
      int p2 = 0;
      int p3 = 0;

      while (height1 != height2 || height1 != height3)
      {
         if (height1 >= height2 && height1 >= height3)
            height1 -= h1[p1++];
         else if (height2 >= height1 && height2 >= height3)
            height2 -= h2[p2++];
         else
            height3 -= h3[p3++];
      }

      return height1;
   }
}

Answer 2

You can check the top solutions from the other users.

Answer 3

我建议从最高堆栈中移除圆柱体，而堆栈高度不同：

// Assuming that we don't have cylinders of negative height
public static int equalStacks(List<int> h1, List<int> h2, List<int> h3) {
  if (h1.Count <= 0 || h2.Count <= 0 || h3.Count <= 0)
    return 0;

  int height1 = h1.Sum();
  int height2 = h2.Sum();
  int height3 = h3.Sum();

  int p1 = 0;
  int p2 = 0;
  int p3 = 0;

  while (height1 != height2 || height1 != height3) {
    if (height1 >= height2 && height1 >= height3) 
      height1 -= h1[p1++];
    else if (height2 >= height1 && height2 >= height3) 
      height2 -= h2[p2++];
    else 
      height3 -= h3[p3++];
  }

  return height1; 
}