valid_perfect_square.cpp

class Solution {
public:
    bool isPerfectSquare(int num) {
        long i = 1, j = num;
        while (i <= j) {
        	long mid = i + (j-i)/2;
        	if (mid*mid == num) return true;
        	if (mid*mid < num) {
        		i = mid + 1;
        	}else {
        		j = mid - 1;
        	}
        }
        return false;
    }
};