方法一：

思路：

二分查找：low high mid均定義為long類型，否則會溢出。

public class Solution {    public boolean isPerfectSquare(int num) {        long low = 1;        long high = num;        while (low <= high) {            long mid = low + (high - low) / 2;             if (mid * mid == num)                return true;            else if (mid * mid < num)                low = mid + 1;            else                high = mid - 1;        }        return false;    }}
Runtime：0ms
方法二：
思路：
public class Solution {    public boolean isPerfectSquare(int num) {        for (int i = 1; i <= num / i; i++) {            if (i * i == num)                 return true;        }        return false;    }}
Runtime：3ms