367. Valid Perfect Square 难度：medium

题目：

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: 16Returns: True

Example 2:

Input: 14Returns: False

思路：

二分查找即可。

程序：

class Solution {public:    bool isPerfectSquare(int num) {        if(num < 1) return false;          if(num == 1) return true;          int left = 0, right = num / 2;          while(left <= right)          {              long mid = (left+right)/2;              long val = mid*mid;              if(val == num) return true;              else if(val > num) right = mid-1;              else left = mid+1;          }          return false;      }};