LeetCode [889] Construct Binary Tree from Preorder and Postorder Traversal

 889. Construct Binary Tree from Preorder and Postorder Traversal

Medium

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:

• 1 <= pre.length == post.length <= 30
• pre[] and post[] are both permutations of 1, 2, ..., pre.length.
• It is guaranteed an answer exists. If there exists multiple answers, you can return any of them.

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/** * 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; * } * } */ //build the tree by pre: 1) root, 2) left, 3) right //when root.vall==post[postIndex], we've done build tree root for post[0..postIndex] //using the post's order to tell when the tree is completed class Solution { int preIndex = 0, postIndex = 0; public TreeNode constructFromPrePost(int[] pre, int[] post) { TreeNode root = new TreeNode(pre[preIndex++]); if(root.val != post[postIndex]){ root.left = constructFromPrePost(pre, post); } if(root.val != post[postIndex]){ root.right = constructFromPrePost(pre, post); } postIndex++; return root; } }