二分查找算法

704. 二分查找

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

在排序数组中查找元素的第一个和最后一个位置

class Solution {
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        if(nums.size() == 0)
            return {-1, -1};
        vector<int> ret;
        // 找左
        int left = 0;
        int right = nums.size() - 1;
        while (left != right)
        {
            int mid = (right + left) / 2; // 落在左边
            if (nums[mid] < target)
                left = mid + 1;
            else if (nums[mid] >= target)
                right = mid;
        }
        if(nums[left] == target)
            ret.push_back(left);
        else 
            ret.push_back(-1);
        // 找右
        left = 0;
        right = nums.size() - 1;
        while (left != right)
        {
            int mid = (right + left + 1) / 2; // 落在右边
            if (nums[mid] <= target)
                left = mid;
            else if (nums[mid] > target)
                right = mid - 1;
        }
        if(nums[left] == target)
            ret.push_back(left);
        else 
            ret.push_back(-1);
        return ret;
    }
};

69. x的平方根

class Solution {
public:
    int mySqrt(int x) {
        if(x == 0)
            return 0;
        int left = 1;
        int right = x;
        while(left != right)
        {
            long long mid = left + (right - left + 1) / 2; // 防止溢出
            if(mid * mid <= x)
                left = mid;
            else if(mid * mid > x)
                right = mid - 1;
        }
        return left;
    }
};