LeetCode:Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[  [2,4],  [3,4],  [2,3],  [1,2],  [1,3],  [1,4],]

这题目实现需要点小技巧，以前在ACM题中见过。

public ArrayList<ArrayList<Integer>> combine(int n, int k) {ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();if (k <= 0) {return result;}int[] kAarray = new int[k];int i = 0;kAarray[0] = 0;while (i >= 0) {kAarray[i]++;if (kAarray[i] > n) {kAarray[i] = 0;i--;} else {if (i == k - 1) {ArrayList<Integer> list = new ArrayList<Integer>(k);for (int kk : kAarray) {list.add(kk);}result.add(list);} else {kAarray[i+1] = kAarray[i];i++;}}}return result;}