【算法】【並查集】Leetcode高頻面試題

無向圖中連通分量的數目

題目鏈接: https://leetcode-cn.com/problems/number-of-connected-components-in-an-undirected-graph/

class Solution {
public:
    vector<int> parent;
    int countComponents(int n, vector<vector<int>>& edges) {
        for(int i = 0; i < n; i++)  
            parent.push_back(i);
        
        int cnt = n;

        for(int i = 0; i < edges.size(); i++)
        {
            int a = edges[i][0], b = edges[i][1];
            cnt -= merge(a, b);
        }
        return cnt;
    }

    int find(int x)
    {
        int root = x;
        while(root != parent[root])
            root = parent[root];
        return root;
    }

    int merge(int a, int b)
    {
        int p_a = find(a);
        int p_b = find(b);
        if(p_a != p_b) 
            parent[p_a] = p_b;
        return p_a != p_b;
    }

};

島嶼數量

題目鏈接：https://leetcode-cn.com/problems/number-of-islands/

class Solution {
public:
    vector<int> parent;
    int numIslands(vector<vector<char>>& grid) {
        if(!grid.size()) return 0;

        int n = grid.size(), m = grid[0].size();
        for(int i = 0; i < n * m; i++)
            parent.push_back(i);
        
        int cnt = 0;
        for(int i = 0; i < n; i++)
            for(int j = 0; j < m; j++)
                if(grid[i][j] == '1') cnt++;


        for(int i = 0; i < n; i++)
        {
            for(int j = 0; j < m; j++)
            {
                if(i > 0 && grid[i][j] == '1' && grid[i - 1][j] == '1')
                {
                        cnt -= merge(i * m + j, (i - 1) * m + j);
                }
                if(j > 0 && grid[i][j] == '1' && grid[i][j - 1] == '1')
                {
                        cnt -= merge(i * m + j, i * m + j - 1);

                }
            }
        }
        return cnt;
    }

    int find(int x)
    {
        int root = x;
        while(parent[root] != root)
            root = parent[root];
        return root;
    }

    int merge(int a, int b)
    {
        int p_a = find(a);
        int p_b = find(b);
        if(p_a != p_b)
            parent[p_a] = p_b;   
        return p_a != p_b;
    }

};

最長連續序列

題目鏈接：https://leetcode-cn.com/problems/longest-consecutive-sequence/

class Solution {
public:
    unordered_map<int, int> parent, cnt;
    int longestConsecutive(vector<int>& nums) {
        if(!nums.size()) return 0;
        for(int i : nums)
        {
            parent[i] = i;
            cnt[i] = 1;
        }
        int ans = 1;
        for(int num : nums)
        {
            if(parent.count(num + 1))
                ans = max(ans, merge(num, num + 1));
        }
        return ans;
    }

    int find(int x)
    {
        int root = x;
        while(parent[root] != root)
            root = parent[root];
        return root;
    }

    int merge(int a, int b)
    {
        int p_a = find(a);
        int p_b = find(b);
        if(p_a == p_b) 
            return cnt[a];
        parent[p_b] = p_a;
        cnt[p_a] += cnt[p_b];
        return cnt[p_a];
    }
};

被圍繞的區域