前缀和

原理(一般情况):sum[start,end] = sum[end] - sum[start-1];
!!!记得考虑start == 0 的情况：sum [start,end] = sum[0,end] = sum[end];

用前缀和来优化还是挺香的
就比如:
（本题思路为滑动窗口+前缀和）

上面为前缀和优化之后的(差别太大了)

class Solution {
public:
    int minSubArrayLen(int target, vector<int>& nums) {
        int sum = 0;
        int left = 0;
        int right = 0;
        int result = 0;
        int minresult = 0; 
        //一般情况
        while(right < nums.size()){
            sum += nums[right];
            while(sum >= target){//满足条件
                //更新最小长度
                result = right - left + 1;
                if(minresult == 0 || result < minresult){
                    minresult = result;
                }
                sum -= nums[left];
                left++;
            }
            right++;
        }
        return minresult;
    }
};

class Solution {
    public:
    int minSubArrayLen(int target, vector<int>& nums) {
        int sum = 0;
        int left = 0;
        int right = 0;
        int result = 0;
        int minresult = 0;
        //一般情况
        while(right < nums.size()){
            sum = 0;
            for(int i = left; i <= right; i++){
                sum += nums[i];
            }
            while(sum >= target){//满足条件
                //更新最小长度
                result = right - left + 1;
                if(minresult == 0 || result < minresult){
                    minresult = result;
                }
                left++;
                sum = 0;
                for(int i = left; i <= right; i++){
                    sum += nums[i];
                }
            }
            right++;
        }
        return minresult;
    }
};