PattySung

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]

<解題>


class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        HashMap<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < inorder.length; i++){
            map.put(inorder[i], i);
        }
        return helper(preorder, inorder, 0, 0, inorder.length - 1, map);
    }
    private TreeNode helper(int[] preorder,
                            int[] inorder,
                            int preStart,
                            int inStart,
                            int inEnd,
                            HashMap<Integer, Integer> map){
        if(preStart >= preorder.length || inStart > inEnd) return null;
        TreeNode root = new TreeNode(preorder[preStart]);

        int rootIndex = map.get(preorder[preStart]);

        root.left = helper(preorder, inorder, preStart+1, inStart, rootIndex - 1, map);
        root.right = helper(preorder, inorder, preStart + rootIndex - inStart + 1, rootIndex + 1, inEnd, map);
        
        return root;
    }
}

Time: O(n) Space: O(n)

Leetcode-105. Construct Binary Tree from Preorder and Inorder Traversal

<解題>