367. Valid Perfect Square

num, write a function which returns True if num

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

Example 1:

Input: 16 Returns: True

Example 2:

注意 mid 和 tmp都必须是long long 防止tmp溢出！

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