367. Perfect Valid Square [LeetCode]

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

Note: Do not use any built-in library function such assqrt.

Example 1:

Input: 16
Returns: True

Example 2:

Input: 14
Returns: False

• 涉及平方数的时候要考虑int(2^32)是否够用，尽量用long(2^64)
• Math.pow会自动生成double型
• long和int可以直接比较不需要cast

class Solution {
    public boolean isPerfectSquare(int num) {
        long start = 1;
        long end = num / 2 + 1;
        long mid;
        while (start + 1 < end) {
            mid = start + (end - start) / 2;
            if (mid * mid < num) {
                start = mid;
            } else if (mid * mid > num) {
                end = mid;
            } else {
                return true;
            }
        }
        if (start * start == num || end * end == num) {
            return true;
        }
        return false;
    }
}