LeetCode 106. Construct Binary Tree from Inorder and Postorder Traversal

Question

Given two integer arrays inorder and postorder where inorder is the inorder traversal of a binary tree and postorder is the postorder traversal of the same tree, construct and return the binary tree.

Example 1:

Input: inorder = [9,3,15,20,7], postorder = [9,15,7,20,3]
Output: [3,9,20,null,null,15,7]

Example 2:

Input: inorder = [-1], postorder = [-1]
Output: [-1]

Solution

• Solution1 – postorder traversal, inorder

class Solution {
    int[] inorder;
    int[] postorder;
    int N;
    Map<Integer, Integer> inMap = new HashMap<>();
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        this.inorder = inorder;
        this.postorder = postorder;
        N = inorder.length;
        for (int i = 0; i < N; i++) inMap.put(inorder[i], i);
        return helper(0, N - 1, 0, N - 1);
    }
    public TreeNode helper(int inStart, int inEnd, int postStart, int postEnd){
        if (inStart > inEnd) return null;
        TreeNode root = new TreeNode(postorder[postEnd]);
        int inIndex = inMap.get(root.val);
        int rightTreeSize = inEnd - inIndex;
        root.left = helper(inStart, inIndex - 1, postStart, postEnd - rightTreeSize -1);
        root.right = helper(inIndex + 1, inEnd, postEnd - rightTreeSize, postEnd -1);
        return root;
    }
}

Complexity:

Time complexity: O(n)

Space complexity: O(n)