209 Minimum Size Subarray Sum

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