LeetCode #862 Shortest Subarray with Sum at Least K

题目

Shortest Subarray with Sum at Least K

Return the length of the shortest, non-empty, contiguous subarray of A with sum at least K.

If there is no non-empty subarray with sum at least K, return -1.

Example 1:

Input: A = [1], K = 1
Output: 1

Example 2:

Input: A = [1,2], K = 4
Output: -1

Example 3:

Input: A = [2,-1,2], K = 3
Output: 3

Note:

1. 1 <= A.length <= 50000
2. -10 ^ 5 <= A[i] <= 10 ^ 5
3. 1 <= K <= 10 ^ 9

Difficulty: Hard

分析过程

这个问题要求出所给数组中，所有数之和至少为K的子数组的最短长度。

暴力求解

建立 long int 类型的数组 sums ，sums[i] 表示前 i 个数的和。那么，就可以由 sums[j] - sums[i] 求出从第 i + 1 个数到第 j 个数的和了。这样只要遍历一次源数组，比对每个数对 ( i , j ) 都相加这个区间内的若干个数要高效。

然后，对每个数对 ( i , j ) ，都检查 sums[j] - sums[i] >= K 是否成立。时间复杂度是 $O(N$