Construct Binary Tree from Preorder and Postorder Traversal

Return any binary tree that matches the given preorder and postorder traversals.

Values in the traversals pre and post are distinct positive integers.

 

Example 1:

Input: pre = [1,2,4,5,3,6,7], post = [4,5,2,6,7,3,1]
Output: [1,2,3,4,5,6,7]

 

Note:


Solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    // Input: pre = [1,2,4,5,3,6,7], post = [4,5,2,6,7,3,1]
    
    // Output: [1,2,3,4,5,6,7]
    public TreeNode constructFromPrePost(int[] pre, int[] post) {
        // System.out.println(Arrays.toString(pre) + ", " + Arrays.toString(post));
        if (pre.length == 0) return null;
        if (pre.length == 1) return new TreeNode(pre[0]);
        int[] postMap = new int[31];
        for (int i = 0; i < post.length; i ++) {
            postMap[post[i]] = i;
        }
        TreeNode root = new TreeNode(pre[0]);
        int leftLen = postMap[pre[1]] + 1;
        root.left = constructFromPrePost(Arrays.copyOfRange(pre, 1, leftLen + 1), Arrays.copyOfRange(post, 0, leftLen));
        root.right = constructFromPrePost(Arrays.copyOfRange(pre, 1 + leftLen, pre.length), Arrays.copyOfRange(post, leftLen, post.length - 1));
        return root;
    }
}