暴力

class Solution {
public:
    bool isPerfectSquare(int num) {
        for(long i = 1; i * i <= num; i++) {
            if(i * i == num) return true;
        }
        return false;
    }
};

二分查找

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