关于c++:C-STL中的Binary-search二分查找

左闭右开写法

• 查找某个元素是否呈现。

int binarySearch(vector<int>& nums, int target)
{
    int left = 0;
    int right= nums.size();
    int mid = 0;
    //[left, mid) mid [mid+1, right)
    while(left< right)
    {mid = (right - left) / 2 + left;
        if(nums[mid] == target) return mid;
        if(nums[mid] < target) left = mid + 1;
        if(nums[mid] > target) right = mid;      
    }
    return -1; // 未找到 target
}

• 查找第一个大于或等于某个元素的地位

int lowerBound(vector<int> &nums, int target) {
    int left = 0;
    int right = nums.size();
    int mid = 0;
    while(left < right)
    {mid = (right - left) / 2 + left;
        if (nums[mid] >= target) {right = mid;} else {left = mid + 1;}
    }
    return left;
}

• 查找第一个大于某个元素的地位。

int upperBound(vector<int> &nums, int target) {
    int left = 0;
    int right = nums.size();
    int mid = 0;
    while(left < right)
    {mid = (right - left) / 2 + left;
        if (nums[mid] > target) {right = mid;} else {left = mid + 1;}
    }
    return left;
}