367.Valid Perfect Square

Tags: Easy Math Binary Search

Links: https://leetcode.com/problems/valid-perfect-square/

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Given a positive integer num, write a function which returns True if num is a perfect square else False.

Note: Do not use any built-in library function such as sqrt.

Example 1:

Input: 16
Output: true

Example 2:

Input: 14
Output: false

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class Solution {
public:
    bool isPerfectSquare(int num) {
        int left = 0, right = num;
        while (left <= right) {
            long long mid = left + ((right - left) >> 1);
            long long res = mid * mid;
            if (res == num) return true;
            else if (res < num) left = mid + 1;
            else right = mid - 1;
        }

        return false;
    }
};

本质仍然是二分查找中的第一类，查找元素是否存在。

方法二：

class Solution {
public:
    bool isPerfectSquare(int num) {
        int i=1;

        while(i<=num/i)
            i++;
        i--;
        return i*i==num;
    }
};

上面的方法还需要类型转换，因为会存在mid * mid溢出的情况，为了避免这种情况，实际上就是从最小的数开始一个个的搜索，直到找到第一个大于num的数，用它前一个数去判断是否满足。