77. Combinations

题目

Given two integers n and k, return all possible combinations of k numbers out of 1 … n.

For example,
If n = 4 and k = 2, a solution is:

[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]

思路

本题是一个典型的组合问题，第一种思路是递归的思想，代码如下：

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

第二种是官方的解法，很巧妙，如下：

class Solution {public:    vector<vector<int>> combine(int n, int k) {        vector<vector<int>> res;        vector<int> tmp(k, 0);        int i = 0;        while (i >= 0) {            ++tmp[i];            if (tmp[i] > n) --i;            else if (i == k - 1) res.push_back(tmp);            else {                ++i;                tmp[i] = tmp[i - 1];            }        }        return res;    }};