LeetCode: Shortest Subarray with Sum at Least K

// https://code.dennyzhang.com/shortest-subarray-with-sum-at-least-k
// Basic Ideas: presum + monotone queue
//
// Get a presum array
//  If current value is smaller than previous value, 
//     for the following values,
//     no need to compare with previous values any more
//
// Complexity: Time O(n), Space O(n)
func shortestSubarray(A []int, K int) int {
    presums := make([]int, len(A)+1)
    for i, v := range A {
        presums[i+1] = presums[i] + v
    }
    res := 1<<31-1
    // find next +K greater value
    // items in queue are increassing
    queue := []int{}
    for i, v := range presums {
        // previous value can't be candidates
        for len(queue)>0 && v <= presums[queue[len(queue)-1]] {
            // remove the item
            queue = queue[0:len(queue)-1]
        }
        // find candidates
        for len(queue)>0 && presums[queue[0]] + K <= v {
            j := queue[0]
            queue = queue[1:]
            if i-j < res {
                res = i-j
            }
        }
        // put current element to be evaluated
        queue = append(queue, i)
    }
    if res == 1<<31-1{
        res = -1
    }
    return res
}