105.construct-binary-tree-from-preorder-and-inorder-traversal.java

import java.util.HashMap;
import java.util.Map;

import javax.swing.tree.TreeNode;

/*
 * @lc app=leetcode id=105 lang=java
 *
 * [105] Construct Binary Tree from Preorder and Inorder Traversal
 */

// @lc code=start
/**
 * 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 {
// int n;
// Map<Integer, Integer> indexMap;
// int[] inorder, preorder;

// public TreeNode buildTree(int[] preorder, int[] inorder) {
// n = inorder.length;
// indexMap = new HashMap<>();
// this.inorder = inorder;
// this.preorder = preorder;

// for (int i = 0; i < n; i++) {
// indexMap.put(inorder[i], i);
// }

// return buildTreePreIn(0, n - 1, 0, n - 1);
// }

// public TreeNode buildTreePreIn(int preStart, int preEnd, int inStart, int
// inEnd) {
// if (inStart > inEnd) {
// return null;
// }

// int preRoot = preStart;
// int inRoot = indexMap.get(preorder[preRoot]);

// TreeNode root = new TreeNode(preorder[preRoot]);
// int leftSubtreeSize = inRoot - inStart;

// root.left = buildTreePreIn(preStart + 1, preStart + leftSubtreeSize, inStart,
// inRoot - 1);
// root.right = buildTreePreIn(preStart + leftSubtreeSize + 1, preEnd, inRoot +
// 1, inEnd);
// return root;
// }
// }

class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        if (preorder == null || preorder.length == 0) {
            return null;
        }
        TreeNode root = new TreeNode(preorder[0]);
        Deque<TreeNode> stack = new LinkedList<TreeNode>();
        stack.push(root);

        int inorderIndex = 0;
        for (int preorderIndex = 1; preorderIndex < preorder.length; preorderIndex++) {
            TreeNode node = stack.peek();

            if (node.val != inorder[inorderIndex]) {
                node.left = new TreeNode(preorder[preorderIndex]);
                stack.push(node.left);
            } else {
                while (!stack.isEmpty() && stack.peek().val == inorder[inorderIndex]) {
                    node = stack.pop();
                    inorderIndex++;
                }
                node.right = new TreeNode(preorder[preorderIndex]);
                stack.push(node.right);
            }
        }
        return root;
    }
}

// @lc code=end