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Input: 16
Returns: True

Input: 14
Returns: False

思路：

二分查找。

java code:

public class Solution {
    public boolean isPerfectSquare(int num) {
        
        long lo = 1, hi = num / 2;
        
        if(num == 1) return true;
        
        long x = num;
        
        while(lo <= hi) {
            long mid = lo + (hi - lo) / 2;
            if(mid * mid == x)
                return true;
            else if(mid * mid < x)
                lo = mid + 1;
            else
                hi = mid - 1;
        }
        return false;
    }
}

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