[LeetCode] 77. Combinations 全组合

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],
]

给两个数字n, k，返回所有由[1...n]中的k数字组合的可能解。

解法1: 递归

解法2: 迭代

C++: Recursion

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

C++: Iteration

class Solution {
public:
    vector<vector<int> > permute(vector<int> &num) {
        vector<vector<int> > res;
        permuteDFS(num, 0, res);
        return res;
    }
    void permuteDFS(vector<int> &num, int start, vector<vector<int> > &res) {
        if (start >= num.size()) res.push_back(num);
        for (int i = start; i < num.size(); ++i) {
            swap(num[start], num[i]);
            permuteDFS(num, start + 1, res);
            swap(num[start], num[i]);
        }
    }
};

　　

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