LintCode 15. Permutations

典型搜索题，搜索题主要有两个，Subsets子集和Permutation全排列。

模板题，递归搜索，要注意记忆模板，遇到相似题改写即可。

class Solution {
public:
    /*
     * @param nums: A list of integers.
     * @return: A list of permutations.
     */
    vector<vector<int>> permute(vector<int> &nums) {
        vector<vector<int> > result;
        vector<int> permutation;
        if (nums.empty()) {
            result.push_back(permutation);
            return result;
        }
        
        
        vector<bool> used(nums.size(), false);
        helper(result, permutation, nums, used);
        
        return result;
    }
    
    void helper(vector<vector<int>> &result,
                vector<int> &permutation,
                vector<int> &nums,
                vector<bool> used) {
        if (permutation.size() == nums.size()) {
            result.push_back(permutation);
            return;
        }
        for (int i = 0; i < nums.size(); i++) {
            if (used[i]) {
                continue;
            }
            used[i] = true;
            permutation.push_back(nums[i]);
            helper(result, permutation, nums, used);
            permutation.pop_back();
            used[i] = false;
        }
    }
};