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.

解法 :
         基本上就是動態規劃的精神，很難口語描述，配合程式碼一起講。

class Solution {
public:
    int minSubArrayLen(int s, vector<int>& nums) {
        
        int n = nums.size(), start = 0, sum = 0, minlen = INT_MAX;
        
        for (int i = 0; i < n; i++) 
        { 
            //將此index的值加進來  
            sum += nums[i];
            
            //發現sum的值大於等於 s後進去條件式
            //跟目前發現的最小長度比大小
            //並且把sum的減去現在的起始點的值，再把起始點往前
            //嘗試找滿足條件的最小值
            while (sum >= s) 
            {
                minlen = min(minlen, i - start + 1);
                sum -= nums[start++];
            }
        }
        
        return minlen == INT_MAX ? 0 : minlen;
    }
};