精确二分搜索与非精确二分搜索

　　int l = 0, r = list.size() - 1;
    while(l <= r){
        mid = l + (r - l) / 2;
        if(list[mid] == target) return mid;
        else if(list[mid] < target) l = mid + 1;
        else r = mid - 1;
    }
    return -1;

//mid即为小于tar的最大值的下表
//注意mid为-1和list.size() - 1的时候
int l = 0, r = list.size() - 1;
int mid = l + (r - l) / 2;
while(l <= r){
    mid = l + (r - l) / 2;
    if(list[mid] == target){
        return true;
    }else if(list[mid] < target){
        l = mid + 1;
    }else{
        r = mid - 1;
        if(l > r) mid--;
    }
}