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Number of Islands II

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Description

https://leetcode.com/problems/number-of-islands-ii/description/

Solution

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class Solution {
public:
    vector<int> numIslands2(int m, int n, vector<pair<int, int>>& positions) {
        vector<int> ret;
        vector<int> roots(m * n, -1);
        vector<vector<int>> dirs = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
        int count = 0;
        for (auto& position : positions) {
            int x = position.first;
            int y = position.second;
            int pos = x * n + y;
            if (roots[pos] == -1) {
                roots[pos] = pos;
                count += 1;
            }
            for (auto& dir : dirs) {
                int newX = x + dir[0];
                int newY = y + dir[1];
                int newPos = newX * n + newY;
                if (newX < 0 || newX >= m || newY < 0 || newY >= n || roots[newPos] == -1) continue;
                int rootCur = findRoot(roots, pos);
                int rootNew = findRoot(roots, newPos);
                
                if (rootCur != rootNew) {
                    roots[rootCur] = rootNew;
                    --count;
                }
            }
            ret.push_back(count);
        }
    
        return ret;
    }
    
    int findRoot(vector<int>& roots, int pos) {
        stack<int> st;
        while (roots[pos] != pos) {
            pos = roots[pos];
            st.push(pos);
        }
        while(!st.empty()) {
            roots[st.top()] = pos;
            st.pop();
        }
        return pos;
    }
};