LeetCode-473火柴棍摆成正方形

LeetCode-473火柴棍摆成正方形

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 could make one square using all the matchsticks the little match girl has.

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:

1. The length sum of the given matchsticks is in the range of 0 to 10^9.
2. The length of the given matchstick array will not exceed 15.

解题思路：//利用双向队列，队列中元素从大到小 ，每次从前后找到两个元素，使其能构成一个边，然后将符合条件的元素pop出，如果最后队列为空，说明可以构成一个正方形。

利用一个双向队列，

class Solution3

{
public:
    //利用双向队列，队列中元素从大到小，
    //每次从前后找到两个元素，使其能构成一个边，然后将符合条件的元素pop出
    //如果最后队列为空，说明可以构成一个正方形
    bool makesquare(vector<int> &nums)
    {
        if(nums.size() < 4)
            return false;


        sort(nums.begin(), nums.end(), cmp);   //将元素从大到小排列
        deque<int> Q;
        int sum = 0;                //火柴棍的总和
        for(int i = 0; i < nums.size(); i++)
        {
            sum += nums[i];
            Q.push_back(nums[i]);
        }
        if(sum % 4 != 0)            //如果sum不是4的倍数，则不能构成正方形
            return false;
        int target = sum / 4;       //正方形的边长
        while(!Q.empty())
        {
            int value = Q.front();
            Q.pop_front();
            while(value < target)   //如果边不够target，则从小的一端拿出一端，直到满足
            {
                value += Q.back();
                Q.pop_back();
            }
        }
        return Q.empty();
    }
};