permutations

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

第一位和2,3,..n-1互换， 固定第一， 2和3...n-1互换。。。直到第n-1位固定，就是所有的组合了。。。

class Solution {public:vector<vector<int> > permute(vector<int> &num) {vector<vector<int> > res;permute(num, 0, num.size() - 1, res);return res;}void permute(vector<int> &input, int k, int n, vector<vector<int> > &res) {if (k == n) {res.push_back(input);} else {for (int i = k; i <= n; i++) {int tmp = input[i];input[i] = input[k];input[k] = tmp;permute(input, k + 1, n, res);input[k] = input[i];input[i] = tmp;}}}};