77组合

给定两个整数 n 和 k，返回 1 ... n 中所有可能的 k 个数的组合。

示例:

输入: n = 4, k = 2
输出:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

最容易想到的应该是回溯，更多题目思路及代码见：https://github.com/CallmeZhouxiaolun/leetcode/tree/master/ordered/71to80

class Solution {
public:
    vector> combine(int n, int k) {
        vector> res;
        vector cb;
        backtracking(res, cb, n, k,1);
        return res;
    }
    void backtracking(vector>& res, vector& cb, int n, int k, int c){
        if(cb.size() == k){
            res.push_back(cb);
            return;
        }
        for(int i=c; i<=n; i++){
            cb.push_back(i);
            backtracking(res, cb, n, k, i+1);
            cb.pop_back();
        }
    }
};

后来看到大佬的做法，有迭代方法，

class Solution {
public:
    vector> combine(int n, int k) {
        vector> res;
        vector cb;
        backtracking(res, cb, n, k,1);
        return res;
    }
    void backtracking(vector>& res, vector& cb, int n, int k, int c){
        if(cb.size() == k){
            res.push_back(cb);
            return;
        }
        for(int i=c; i<=n; i++){
            cb.push_back(i);
            backtracking(res, cb, n, k, i+1);
            cb.pop_back();
        }
    }
};