using System; using System.Collections.Generic; using System.Linq; using System.Threading.Channels; namespace CombinationAll { public class Program { private List<string> _input = new List<string>() { "a", "b", "c", "d" // ,"e","f","g","h","i", }; private static void Main(string[] args) { var program = new Program(); // NSelectOne(ref program._input).ForEach(Console.WriteLine); // NSelectTwo(ref program._input).ForEach(Console.WriteLine); // NSelectThree(ref program._input).ForEach(Console.WriteLine); // System.Console.WriteLine("==============================="); // NSelectM(ref program._input, 1).ForEach(Console.WriteLine); // NSelectM(ref program._input, 2).ForEach(Console.WriteLine); // NSelectM(ref program._input, 3).ForEach(Console.WriteLine); // System.Console.WriteLine("==============================="); for (var i = 0; i < program._input.Count; i++) { Console.WriteLine($"input,{program._input.Count}选{i + 1}"); NSelectM(ref program._input, i + 1).ForEach(Console.WriteLine); } Console.WriteLine("---->over"); } /// <summary> /// KeyMethod /// </summary> /// <param name="input">原列表</param> /// <param name="m">选几个</param> /// <returns>结果</returns> /// <exception cref="Exception">有bug</exception> private static List<string> NSelectM(ref List<string> input, int m) { if (m > input.Count) { throw new Exception("n选m，m必须小于n!"); } List<string> result = new List<string>(); Stack<int> indexStack = new Stack<int>(); GoDeepFor(ref input, m, -1, ref result, ref indexStack); return result; static void GoDeepFor(ref List<string> input, in int m, in int preIndex, ref List<string> result, ref Stack<int> indexStack) { for (int index = preIndex + 1; index < input.Count; index++) { if (index > input.Count) break; indexStack.Push(index); if (indexStack.Count >= m) { var str = ""; foreach (int num in indexStack.Reverse()) { str += input[num]; } result.Add(str); indexStack.Pop(); } else { GoDeepFor(ref input, m, index, ref result, ref indexStack); indexStack.Pop(); } } } } #region NoNeed private static List<string> NSelectThree(ref List<string> input) { List<string> result = new List<string>(); for (var i = 0; i < input.Count; i++) { if (i > input.Count) break; for (int j = i + 1; j < input.Count; j++) { if (j > input.Count) break; for (int k = j + 1; k < input.Count; k++) { if (k > input.Count) break; result.Add(input[i] + input[j] + input[k]); } } } return result; } private static List<string> NSelectTwo(ref List<string> input) { List<string> result = new List<string>(); for (var i = 0; i < input.Count; i++) { if (i > input.Count) break; for (int j = i + 1; j < input.Count; j++) { if (j > input.Count) break; result.Add(input[i] + input[j]); } } return result; } private static List<string> NSelectOne(ref List<string> input) { List<string> result = new List<string>(); for (var i = 0; i < input.Count; i++) { if (i > input.Count) break; result.Add(input[i]); } return result; } #endregion } }input,4选1 a b c d input,4选2 ab ac ad bc bd cd input,4选3 abc abd acd bcd input,4选4 abcd ---->over
优化方向：
1. 递归缓冲；
2.C（5，2）=C（5，3）；
方法+2
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