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<vector<int>> combine(int n, int k) { vector<vector<int>> res; vector<int> cb; backtracking(res, cb, n, k,1); return res; } void backtracking(vector<vector<int>>& res, vector<int>& 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<vector<int>> combine(int n, int k) { vector<vector<int>> res; vector<int> cb; backtracking(res, cb, n, k,1); return res; } void backtracking(vector<vector<int>>& res, vector<int>& 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(); } } };