LeetCode/java/find-minimum-diameter-after-merging-two-trees.java

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.List;

class Solution {
  private List<List<Integer>> makeGraph(int[][] edges) {
    var n = edges.length + 1;

    var g = new ArrayList<List<Integer>>();
    for (int i = 0; i < n; ++i) {
      g.add(new ArrayList<>());
    }

    for (var edge : edges) {
      g.get(edge[0]).add(edge[1]);
      g.get(edge[1]).add(edge[0]);
    }

    return g;
  }

  private int getDiameter(int[][] edges) {
    var n = edges.length + 1;
    var g = makeGraph(edges);
    var deg = new int[n];

    var q = new ArrayDeque<Integer>();
    for (int i = 0; i < n; ++i) {
      deg[i] = g.get(i).size();
      if (deg[i] == 1) {
        q.offer(i);
      }
    }

    int levelsRemoved = 0;

    int remaining = n;
    while (remaining > 2) {
      var size = q.size();
      remaining -= size;
      ++levelsRemoved;

      for (int i = 0; i < size; ++i) {
        var u = q.poll();

        for (var v : g.get(u)) {
          --deg[v];
          if (deg[v] == 1) {
            q.offer(v);
          }
        }
      }
    }

    return 2 * levelsRemoved + ((remaining == 2) ? 1 : 0);
  }

  public int minimumDiameterAfterMerge(int[][] edges1, int[][] edges2) {
    var d1 = getDiameter(edges1);
    var d2 = getDiameter(edges2);
    var combined = (d1 / 2 + d1 % 2) + (d2 / 2 + d2 % 2) + 1;

    return Math.max(combined, Math.max(d1, d2));
  }
}