自己写的AC代码：

class Solution {
public:
    int countBalls(int lowLimit, int highLimit) {
        vector<int> cnt(highLimit+1,0);
        int res = 0;
        for(int i = lowLimit;i <= highLimit;i++)
        {
            cnt[calsum(i)]++;
        }
        
        for(int i = 0;i < cnt.size();i++)
        {
            res = max(res,cnt[i]);
        }
        
        return res;
    }
private:
    int calsum(int num){
        int res = 0;
        while(num){
            res += num %10;
            num /= 10;
        }
        
        return res;
    }
};

运行效率:

Runtime: 24 ms, faster than 59.07% of C++ online submissions for Maximum Number of Balls in a Box.
Memory Usage: 13.3 MB, less than 5.21% of C++ online submissions for Maximum Number of Balls in a Box.

存在几点优化的点：
1.输入数据1 <= lowLimit <= highLimit <= 10的5次方，因此求和最大99999 ----> 5*9 =45；
2.求数组 vector等最大值stl存在方法,性能更高
优化后的代码：

class Solution {
public:
    int countBalls(int lowLimit, int highLimit) {
        int cnt[46] = {0};
        for(int i = lowLimit;i <= highLimit;i++)
        {
            int n = i,sum = 0;
            while(n)
            {
                sum += n%10;
                n /= 10;
            }
            cnt[sum]++;
        }
        
        return *max_element(begin(cnt),end(cnt));
    }
};

运行效率:

Runtime: 8 ms, faster than 93.52% of C++ online submissions for Maximum Number of Balls in a Box.
Memory Usage: 5.9 MB, less than 94.01% of C++ online submissions for Maximum Number of Balls in a Box.