【C++】经典二分

class Solution {
public:
    int search(vector<int>& nums, int target) {
        return(helper(nums, target, 0, nums.size() - 1));
    }
    int helper(vector<int>& nums, int &target, int left, int right) {
        if(left > right || nums[left] > target || nums[right] < target) //边界检测
            return -1;
        if(nums[left] == target) //判断一个区间最左边的值是否为target
            return left;
        int mid = (left + right) / 2; //以中点分为两个区间
        int l_i = helper(nums, target, left, mid); //递归得到左区间target的索引值
        if(l_i != -1) //不为-1说明找到索引
            return l_i;
        return helper(nums, target, mid + 1, right); //若左区间没找到则找右区间
    }
};