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

答案

class Solution {
    public boolean isPerfectSquare(int num) {
        long l = 0, r = num+1;
        while(l < r) {
            long m = (l + r) / 2;
            long prod = m*m;
            if(prod == num) {
                return true;
            }
            if(prod < num) {
                l = m + 1;
            }
            else {
                r = m;
            }
        }
        return false;
    }
}