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.

click to show more practice.

More practice:
If you have figured out the O(n) solution, try coding another solution of which the time complexity is O(n log n).
两指针问题，动态决定前后指针的位置
*/

class Solution {
public:
int minSubArrayLen(int s, vector<int>& nums) {
int sum=0,start=0,end=0,minLen=INT_MAX;
while(end< nums.size())
{
sum+=nums[end++];
while(sum>=s)
{
minLen = min(minLen,end-start);
sum-=nums[start++];
}
}
return minLen==INT_MAX ? 0 : minLen;
}
};