3160. Find the Number of Distinct Colors Among the Balls

You are given an integer limit and a 2D array queries of size n x 2.

There are limit + 1 balls with distinct labels in the range [0, limit]. Initially, all balls are uncolored. For every query in queries that is of the form [x, y], you mark ball x with the color y. After each query, you need to find the number of distinct colors among the balls.

Return an array result of length n, where result[i] denotes the number of distinct colors after ith query.

Note that when answering a query, lack of a color will not be considered as a color.

class Solution {
public:
    vector<int> queryResults(int limit, vector<vector<int>>& Q) {
        unordered_map<int,int> c,rc;
        vector<int> res;
        for(auto& q : Q) {
            int b = q[0], col = q[1];
            if(c.count(b)) {
                if(--rc[c[b]] == 0) rc.erase(c[b]);
            }
            c[b] = col;
            rc[col]++;
            res.push_back(rc.size());
        }
        return res;
    }
};