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LeetCode 367. Valid Perfect Square

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Question

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 as sqrt.

Example 1: 

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Input: 16
Returns: True

Example 2: 

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Input: 14
Returns: False

Explanation

本题是查找平方根。Corner Case排除负数。用二分法找到值，注意overflow的问题，不能用乘法，用除法。

Code

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public class Solution {
    public boolean isPerfectSquare(int num) {
        if(num <= 0) return false;
        if (num == 1) return true;
        int start = 0;
        int end = num;
        while (start + 1 < end) {
            int mid = start + (end - start) / 2;
            if (num / mid == mid) {
                start = mid;
            } else if (num / mid < mid) {
                end = mid;
            } else {
                start = mid;
            }
        }
        if ( num*1.0 / start == start) return true;
        if ( num*1.0 / end == end) return true;
        return false;
    }
}