FB面经Prepare: Find Longest Path in a Multi-Tree

给的多叉树, 找这颗树里面最长的路径长度 

解法就是在子树里面找最大的两个（或一个，如果只有一个子树的话）高度加起来。

对于每一个treenode, 维护它的最高的高度和第二高的高度，经过该点的最大路径就是：  最高高度+第二高高度，然后return 最高高度

 

 1 package fbPractise;
 2 
 3 import java.util.*;
 4 
 5 class TreeNode {
 6     int val;
 7     List children;
 8     public TreeNode(int value) {
 9         this.val = value;
10         this.children = new ArrayList();
11     }
12 }
13 
14 
15 public class LongestPathInTree {
16     static int maxLen = 0;
17     
18     public static int findLongestPath(TreeNode node) {
19         findMaxPath(node);
20         return maxLen;
21     }
22     
23     public static int findMaxPath(TreeNode node) {
24         if (node == null) return 0;
25         int heightest1 = 0;
26         int heightest2 = 0;
27         
28         for (TreeNode child : node.children) {
29             int childHeight = findMaxPath(child);
30             
31             if (childHeight > heightest1) {
32                 heightest2 = heightest1;
33                 heightest1 = childHeight;
34             }
35             else if (childHeight > heightest2) {
36                 heightest2 = childHeight;
37             }
38         }
39         maxLen = Math.max(maxLen, 1 + heightest1 + heightest2);
40         return 1 + heightest1;
41     }
42     
43     
44     public static void main(String[] args) {
45         TreeNode node1 = new TreeNode(1);
46         TreeNode node2 = new TreeNode(2);
47         TreeNode node3 = new TreeNode(3);
48         TreeNode node4 = new TreeNode(4);
49         TreeNode node5 = new TreeNode(5);
50         TreeNode node6 = new TreeNode(6);
51         node1.children.add(node2);
52         node1.children.add(node3);
53         node2.children.add(node4);
54         node2.children.add(node5);
55         node5.children.add(node6);
56         
57         int res = findLongestPath(node1);
58         System.out.println(res);
59     }
60     
61     
62 }