LeetCode 209. Minimum Size Subarray Sum

原题链接在这里：https://leetcode.com/problems/minimum-size-subarray-sum/

题目：

Given an array of n positive integers and a positive integer s, find the minimal length of a 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.

题解：

类似Minimum Window Substring.

双指针维护一个window, 当window 的 sum小于target时一直移动window 的右边, 更新sum.

当sum >= s后一直移动window 左边, 更新sum 和 minLength.

最后返回时看minLength是否被更新过，若被更新过就返回minLength, 没有更新过说明没有符合要求的window, 返回0.

Time Complexity: O(n), array至多走两遍.

Space: O(1).

AC Java:

class Solution {
    public int minSubArrayLen(int s, int[] nums) {
        if(nums == null || nums.length == 0 || s <= 0){
            return 0;
        }
        
        int walker = 0;
        int runner = 0;
        int sum = 0;
        int res = Integer.MAX_VALUE;
        while(runner < nums.length){
            sum += nums[runner++];
            while(sum >= s){
                res = Math.min(res, runner-walker);
                sum -= nums[walker++];
            }
        }
        
        return res == Integer.MAX_VALUE ? 0 : res;
    }
}