Leetcode 209. Minimum Size Subarray Sum

Given an array of n positive integers and a positive integer s, find the minimal length of a contiguous subarray of which the sum ≥ s. If there isn't one, return 0 instead.

For example, given the array [2,3,1,2,4,3] and s = 7,
the subarray [4,3] has the minimal length under the problem constraint.

思路：

1. 暴力求解：依次遍历每个元素，寻找以这个元素为起点的连续和大于s的最短长度，时间复杂度O(n2)。
2. 用两个指针，分别指向当前区间首尾，如果当前区间的和大于s，则更新min length。并且尝试不断右移指向首部的指针，直到当前区间的和小于s。最差情况是需要遍历两倍数组长度，时间复杂度O(n).

public int minSubArrayLen(int s, int[] nums) {
    if (nums == null || nums.length == 0) {
        return 0;
    }

    int res = Integer.MAX_VALUE;
    int start = 0, end = 0, curSum = 0;
    while (end < nums.length) {
        curSum += nums[end];
        end++;
        while (curSum >= s) {
            res = Math.min(res, end - start);
            curSum -= nums[start];
            start++;
        }
    }

    return res == Integer.MAX_VALUE ? 0 : res;
}