C++ STL中的Binary search（二分查找）

左闭右开写法

• 查找某个元素是否出现。

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位置

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位置。

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;
}