LeetCode 105. Construct Binary Tree from Preorder and Inorder Traversal

Question

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

Example 1:

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

Example 2:

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

Solution

• Solution1 – preorder traversal, inorder

class Solution {
    int[] preorder;
    int[] inorder;
    int preOrderIndex;
    Map<Integer, Integer> treeMap = new HashMap<>();
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        this.preorder = preorder;
        this.inorder = inorder;
        this.preOrderIndex = 0;
        int N = preorder.length;
        for (int i = 0; i < N; i++) treeMap.put(inorder[i], i);
        return helperbulid(0, N-1);
    }
    public TreeNode helperbulid(int instart, int inend){
        if(instart > inend) return null;
        TreeNode root = new TreeNode(preorder[preOrderIndex++]);
        int index = treeMap.get(root.val);
        root.left = helperbulid(instart, index - 1);
        root.right = helperbulid(index + 1, inend);
        return root;
    }
}

Complexity:

Time complexity: O(n)

Space complexity: O(n)