367 Valid Perfect Square (Easy)

检验完全平方数

Given a positive integernum, write a function which returns True ifnumis a perfect square else False.

Note:Do notuse any built-in library function such assqrt.

Example 1:

Input: 16
Returns: True

Example 2:

Input: 14
Returns: False

Solution:

A square number is 1+3+5+7+...

//The time complexity is O(sqrt(n))
public boolean isPerfectSquare(int num) {
     int i = 1;
     while (num > 0) {
         num -= i;
         i += 2;
     }
     return num == 0;
 }


 //The time complexity is O(log(n))
 public boolean isPerfectSquare(int num) {
        int low = 1, high = num;

        while (low <= high) {
            long mid = (low + high) >>> 1;

            if (mid * mid == num) {
                return true;
            } else if (mid * mid < num) {
                low = (int) mid + 1;
            } else {
                high = (int) mid - 1;
            }
        }
        return false;
    }