209. Minimum Size Subarray Sum (https://leetcode.com/problems/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.

209. Minimum Size Subarray Sum (#1 Two pointer substring).java

// Two pointer solution: min/max substring template
// Time: O(n), 1ms
// Space: O(1), 37.9mb
class Solution {
    public int minSubArrayLen(int s, int[] nums) {
        // Window is [beign, end)
        int begin = 0, end = 0, minLen = nums.length + 1, sum = 0;
        while(end < nums.length) {
            // Expand window to the right
            sum += nums[end++];
            while(sum >= s) {
                // Update min length while it's still valie
                minLen = Math.min(minLen, end - begin);
                // Shrink the window from left
                sum -= nums[begin++];
            }
        }
        
        return minLen == nums.length + 1 ? 0 : minLen;
    }
}

209. Minimum Size Subarray Sum (#1.1 Two Pointers).java

class Solution {
    public int minSubArrayLen(int s, int[] nums) {
        if(nums.length == 0) return 0;
        int l = 0, r = 0, sum = nums[0], ans = nums.length + 1;
        while(true) {
            if(sum >= s) {
                ans = Math.min(ans, r - l + 1);
                sum -= nums[l++];
            } else if(sum < s) {
                ++r;
                if(r >= nums.length) break;  //Check if the r is out of range
                sum += nums[r];
            }
        }
        if(ans == nums.length + 1) ans = 0;
        return ans;
    }
}