算法思想：

• 双指针枚举i,j；类似三数之和

class Solution {
public:
    bool judgeSquareSum(int c) {
        long long  sum=0;
        vector<int> dp;dp.push_back(0);
        long long start=1;
        while(sum < c)
        {
            sum = start *start;
            if(sum>c) break;
            else dp.push_back(sum);
            start++;

        }
        long long l=0,r=dp.size()-1;
        while(l<=r)
        {
            //long long tmp =dp[l] + dp[r]; 会溢出
            if(dp[l] < c-dp[r])
            {
                l++;
                continue;
            }
            else if (dp[l] > c - dp[r])
            {
                r--;
                continue;
            }
            else if(dp[l] == c-dp[r]) return true;
        }
        return false;

    }
};

但注意一点，dp[l] + dp[r]会溢出，所以用减法不会溢出；