1466. Reorder Routes to Make All Paths Lead to the City Zero.md

1466. Reorder Routes to Make All Paths Lead to the City Zero

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There are n cities numbered from 0 to n-1 and n-1 roads such that there is only one way to travel between two different cities (this network form a tree). Last year, The ministry of transport decided to orient the roads in one direction because they are too narrow.

Roads are represented by connections where connections[i] = [a, b] represents a road from city a to b.

This year, there will be a big event in the capital (city 0), and many people want to travel to this city.

Your task consists of reorienting some roads such that each city can visit the city 0. Return the minimum number of edges changed.

It's guaranteed that each city can reach the city 0 after reorder.

Example 1:

Input: n = 6, connections = [[0,1],[1,3],[2,3],[4,0],[4,5]]
Output: 3
Explanation: Change the direction of edges show in red such that each node can reach the node 0 (capital).

Example 2:

Input: n = 5, connections = [[1,0],[1,2],[3,2],[3,4]]
Output: 2
Explanation: Change the direction of edges show in red such that each node can reach the node 0 (capital).

Example 3:

Input: n = 3, connections = [[1,0],[2,0]]
Output: 0

Constraints:

• 2 <= n <= 5 * 10^4
• connections.length == n-1
• connections[i].length == 2
• 0 <= connections[i][0], connections[i][1] <= n-1
• connections[i][0] != connections[i][1]

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Solution

• mine

  • Java

    Runtime: 4 ms, faster than 100.00%，Memory Usage: 62.2 MB, less than 100.00% of Java online submissions

    public int minReorder(int n, int[][] connections) {
      int res = 0;
      int[] t = new int[n];
      t[0] = 1;
      for(int[] c : connections){
        if(t[c[1]] == 0){
          res++;
        }
        t[c[0]] = 1;
        t[c[1]] = 1;
      }
      return res;
    }
    

    pass all the leetcode test case, but not correct if test case like:

    3
    [[1,2],[2,0]]
    
    6
    [[2,3],[3,1],[1,0],[4,0],[4,5]]
    
    3
    [[2,1],[0,2]]
    

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• the most votes

  Runtime: 15 ms, faster than 100.00%, Memory Usage: 55.5 MB, less than 100.00% of Java online submissions

  public static int dfs(List<List<Integer>> g, int curr, boolean[] visited) {
      visited[curr] = true;
      List<Integer> adj = g.get(curr);
      int ans = 0;
      for(int i = 0; i < adj.size(); i++) if(!visited[adj.get(i)]) ans += 1 + dfs(g, adj.get(i), visited);
      return ans;
  }
  public int minReorder(int n, int[][] connections) {
      List<List<Integer>> g = new ArrayList<>();
      for(int i = 0; i < n; i++) g.add(new ArrayList<>());
      int m = connections.length;
      for(int i = 0; i < m; i++) {
          int sv = connections[i][0], ev = connections[i][1];
          g.get(sv).add(ev);
      }
      boolean[] visited = new boolean[n];
      int ans = 0;
      for(int i = 0; i < n; i++) if(!visited[i]) ans += dfs(g, i, visited);
      return ans;
  }