leetcode:Subsets

Given a set of distinct integers, S, return all possible subsets.

Note:

• Elements in a subset must be in non-descending order.
• The solution set must not contain duplicate subsets.

For example,
If S = [1,2,3], a solution is:

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

题目地址： https://oj.leetcode.com/problems/subsets/

解题思路：

这道题做了两种解法。

解法1：

因为从S中每添加一个元素进来，下一层的集合=上一层的集合+每一个上一层的集合的后面加上新添加的那个元素。如下：

0----------[ ]

1----------[ ] [1]

2----------[ ] [1] [2] [1 2]

3----------[ ] [1] [2] [1 2] [3] [1 3] [2 3] [1 2 3]

.........

代码：

class Solution {
public:
    vector > subsets(vector &S) {
        sort(S.begin(),S.end());
		
		vector > ret(1);
		for(int i=0;i tmp=ret[j];
				tmp.push_back(S[i]);
				ret.push_back(tmp);
			}
		}
		return ret;
    }
};

解法2：

用递归+回溯的方法，这个方法可能看代码会比较清晰。参考了别人的代码：

class Solution {
public:
	vector > ret;
    vector > subsets(vector &S) {
        ret.clear();
		sort(S.begin(),S.end());

		vector tmpans;
		dfs(S,0,tmpans);
		return ret;
    }
private:
	void dfs(vector &s,int loc,vector &tmp){
		if(loc==s.size()){
			ret.push_back(tmp);
			return;
		}
		
		tmp.push_back(s[loc]);
		dfs(s,loc+1,tmp);
		tmp.pop_back();
		dfs(s,loc+1,tmp);
	}
};