LeetCode 862. 和至少为 K 的最短子数组（前缀和+deque单调栈）

示例 1：
输入：A = [1], K = 1
输出：1

示例 2：
输入：A = [1,2], K = 4
输出：-1

示例 3：
输入：A = [2,-1,2], K = 3
输出：3
 
提示：
1 <= A.length <= 50000
-10 ^ 5 <= A[i] <= 10 ^ 5
1 <= K <= 10 ^ 9

class Solution {
public:
    int shortestSubarray(vector<int>& A, int K) {
    	vector<int> presum(A.size()+1,0);
    	int i, minLen = INT_MAX, sum = 0;
    	deque<int> q;//存储下标，按照队内presum升序
    	q.push_back(0);//边界条件，前缀和为0，下标为0
    	for(i = 0; i < A.size(); ++i)
    	{
    		presum[i+1] = presum[i] + A[i];//前缀和
    		while(!q.empty() && presum[q.back()] >= presum[i+1])
    			q.pop_back();
    		while(!q.empty() && (presum[i+1]-presum[q.front()] >= K))
    		{
    			minLen = min(minLen, i+1-q.front());
    			q.pop_front();
    		}
    		q.push_back(i+1);
    	}
    	if(minLen == INT_MAX)
    		return -1;
    	return minLen;
    }
};