【双指针】633. 平方数之和

双指针 O(sqrt(n)):

class Solution {
public:
    bool judgeSquareSum(int c) {
        long long left = 0, right = sqrt(c);
        while(left <= right){
            long long sum = left * left + right*right;
            if (sum == c)
                return true;
            else if (sum < c)
                left += 1;
            else
                right -= 1;
        }
        return false;
    }
};