19.2.15 [LeetCode 77] Combinations

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

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

题意

在1到n中取k个数的所有情况

题解

class Solution {
public:
    void build(vector<vector<int>>&ans,vector<int>&now, int n, int k) {
        if (k == 0) {
            ans.push_back(now);
            return;
        }
        int s = 0;
        if (!now.empty())s = now.back();
        for (int i = s + 1; i <= n - k + 1; i++) {
            now.push_back(i);
            build(ans, now, n, k - 1);
            now.pop_back();
        }
    }
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>>ans;
        vector<int>now;
        build(ans, now, n, k);
        return ans;
    }
};