Maximum Gap

class Solution {
public:
    int maximumGap(vector<int> &num) {
        if (num.size() <= 1) return 0;
        int res = -1;
        radixSort(num);
        for(int i = 1; i < num.size(); ++i) {
            res = max(res, num[i] - num[i-1]);
        }
        return res;
    }
    
    void radixSort(vector<int> &num) {
        int maxN = -1;
        for(auto i : num) {
            maxN = max(maxN, i);
        }
        //create 10 buckets, each for a digit
        int base = 10;
        vector< vector<int> > bucket (base);
        vector<int> tmpVec = num; //keep partially sorted list after each iteration
        for (long long i = 1; i <= maxN; i *= 10) {
            for(int j = 0; j < tmpVec.size(); ++j) {
                int digit = (tmpVec[j] / i) % base;
                bucket[digit].push_back(tmpVec[j]);
            }
            //update temp vector
            int m = 0; //index of tmpVec
            for (int j = 0; j < base; ++j) {
                for (size_t k = 0; k < bucket[j].size(); ++k) {
                    tmpVec[m] = bucket[j][k];
                    m++;
                }
                bucket[j].clear();
                
            }
        }
        num = tmpVec;
    }
};