【Leetcode】209. Minimum Size Subarray Sum（求子区间）

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.

Example: 

Input: s = 7, nums = [2,3,1,2,4,3]
Output: 2
Explanation: the subarray [4,3] has the minimal length under the problem constraint.

Follow up:

If you have figured out the O(n) solution, try coding another solution of which the time complexity is O(n log n). 

题目大意：

题目中给出一个目标，和一个数组，求出长度最少的区间满足和>=s。

解题思路：

从第一个位置开始遍历，当找到满足要求时将左标签l向右移动。保存最小距离，再将右标签r向右移动。

class Solution {
public:
    int minSubArrayLen(int s, vector<int>& nums) {
        // O(n) l and r
        int n = nums.size();
        int ans = INT_MAX;
        int l = 0, r = 0;
        int sum = 0;
        while(r<n){
            sum += nums[r];
            while(sum>=s){
                ans = min(ans, r+1-l);
                sum -= nums[l++];
            }
            r++;
        }
        return (ans==INT_MAX)?0:ans;
    }
};