367. 有效的完全平方数

 1 class Solution 
 2 {
 3 public:
 4     bool isPerfectSquare(int num) 
 5     {
 6         if(num == 1) return true;
 7         for(int i = 1;i <= num/2;i ++)
 8         {
 9             if((long long)i * i == num) return true;
10         }
11         return false;
12     }
13 };

 1 class Solution 
 2 {
 3 public:
 4     bool isPerfectSquare(int num) 
 5     {
 6         // 数学定理(1 + 3 + 5 + ... + (2n - 1) = n ^ 2)
 7         int i = 1;
 8         while(num > 0) 
 9         {
10             num -= i;
11             i += 2;
12         }
13         return num == 0;
14     }
15 };