Leetcode 78 - Subsets（枚举）

题意

求一个set的所有子集

思路

算法1

二进制枚举

算法2

回溯法。

代码

//algorithm 1class Solution {public:    vector<vector<int>> subsets(vector<int>& nums) {        int n = nums.size();        vector<vector<int>> ans;        for (int s = 0; s < (1 << n); s++) {            vector<int> v;            for (int i = 0; i < n; i++) {                if (s & (1 << i)) v.push_back(nums[i]);            }            ans.push_back(v);        }        return ans;    }};//algorothm 2class Solution {public:    vector<int> a;    vector<vector<int>> ans;    void dfs(int pos, vector<int>& v) {        if (pos == a.size()) {            ans.push_back(v);            return;        }        v.push_back(a[pos]);        dfs(pos + 1, v);        v.pop_back();        dfs(pos + 1, v);    }    vector<vector<int>> subsets(vector<int>& nums) {        a = nums;        vector<int> v;        dfs(0, v);        return ans;    }};