Leetcode: Construct Binary Tree from Inorder and Postorder Traversal

Given inorder and postorder traversal of a tree, construct the binary tree.

与Construct Binary Tree from Inorder and Preorder Traversal问题非常类似，唯一区别在于这一次确定root的位置由post traversal来确定，为最后一个元素。

 1 /**

 2  * Definition for binary tree

 3  * public class TreeNode {

 4  *     int val;

 5  *     TreeNode left;

 6  *     TreeNode right;

 7  *     TreeNode(int x) { val = x; }

 8  * }

 9  */

10 public class Solution {

11     public TreeNode buildTree(int[] inorder, int[] postorder) {

12         if (inorder.length==0 || postorder.length==0 || inorder.length!=postorder.length) {

13             return null;

14         }

15         int len = inorder.length;

16         HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();

17         for (int i=0; i<inorder.length; i++) {

18             map.put(inorder[i], i);

19         }

20         return helper(postorder, 0, len-1, inorder, 0, len-1, map);

21     }

22     

23     public TreeNode helper(int[] postorder, int posL, int posR, int[] inorder, int inL, int inR, HashMap<Integer, Integer> map) {

24         if (posL>posR || inL>inR) {

25             return null;

26         }

27         TreeNode root = new TreeNode(postorder[posR]);

28         int index = map.get(postorder[posR]);

29         root.left = helper(postorder, posL, posL+index-1-inL, inorder, inL, index-1, map);

30         root.right = helper(postorder, posL+index-inL, posR-1, inorder, index+1, inR, map);

31         return root;

32     }

33 }