完成日期：7月23日

今日總結(jié)：

77. 組合

// 遞歸，好好看看
class Solution {
private:
    vector<vector<int>> result;
    vector<int> path;
    void backtracking(int n, int k, int startIndex){
        if(path.size() == k){
            result.push_back(path);
            return;
        }
        for(int i = startIndex; i<=n-(k-path.size())+1; ++i){
            path.push_back(i);
            backtracking(n, k, i+1);
            path.pop_back();
        }
    }

public:
    vector<vector<int>> combine(int n, int k) {
        backtracking(n, k, 1);
        return result;
    }
};

46. 全排列

class Solution {
private:
    vector<vector<int>> result;
    vector<int> path;
    void backtracking(vector<int>& nums, vector<bool>& used){
        if(path.size()==nums.size()){
            result.push_back(path);
            return;
        }
        for(int i =0; i<nums.size(); ++i){
            if (used[i] == true)  continue;
            used[i] = true;
            path.push_back(nums[i]);
            backtracking(nums, used);
            path.pop_back();
            used[i] = false;         
        }
    }

public:
    vector<vector<int>> permute(vector<int>& nums) {
        result.clear();
        path.clear();
        vector<bool> used(nums.size(), false);
        backtracking(nums, used);
        return result;
    }
};

784. 字母大小寫全排列

先不做