Leetcode Matchsticks to Square 473

Remember the story of Little Match Girl? By now, you know exactly what matchsticks the little match girl has, please find out a way you can make one square by using up all those matchsticks. You should not break any stick, but you can link them up, and each matchstick must be used exactly one time.

Your input will be several matchsticks the girl has, represented with their stick length. Your output will either be true or false, to represent whether you can save this little girl or not.

Example 1:
Input: [1,1,2,2,2]
Output: true

Explanation: You can form a square with length 2, one side of the square came two sticks with length 1.
Example 2:
Input: [3,3,3,3,4]
Output: false

Explanation: You cannot find a way to form a square with all the matchsticks.
Note:
The length sum of the given matchsticks is in the range of 0 to 10^9.
The length of the given matchstick array will not exceed 15.

题目链接

写搜索一直超时，加上一个条件之后a了

class Solution {public:    bool makesquare(vector<int>& nums) {        int len=nums.size();        int sum=0;        for(int i=0;i<len;i++){            sum+=nums[i];        }        if(len==0 || sum%4!=0) return false;        else{            int len[4];            for(int i=0;i<4;i++)                len[i]=0;            return dfs(nums,0,sum/4,len);        }    }    bool dfs(vector<int>& nums,int index,int sum,int len[]){        if(index==nums.size()) return true;        for(int i=0;i<4;i++){            if(len[i]+nums[index]<=sum){                if (i>0 && len[i]==len[i-1]) {                    continue;                }  //加的减枝条件                len[i]+=nums[index];                if(dfs(nums,index+1,sum,len))                    return true;                len[i]-=nums[index];            }        }        return false;    }};