p105

发表于 | 分类于 |

LeetCode p105 Construct Binary Tree from Preorder and Inorder Traversal 题解

1.题目：

Given preorder and inorder traversal of a tree, construct the binary tree.

题意：

输入一二叉树的前续遍历，中序遍历，返回这个二叉树。

2.解题思路:

前续的第一个为头，切分中序，递归。

3.代码

[title] [] [url] [link text]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
 
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
     if (inorder.length == 0)
			return null;
		int len = preorder.length;
		HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();

		for (int i = 0; i < len; i++) {
			map.put(inorder[i], i);
		}

		return buildTree(inorder, 0, len - 1, preorder, 0, len - 1, map);
	}

	public TreeNode buildTree(int[] inorder, int is, int ie,
			int[] preorder, int ps, int pe, HashMap<Integer, Integer> map) {
		if (is > ie || ps > pe)
			return null;
		TreeNode node = new TreeNode(preorder[ps]);
		System.out.println(preorder[ps]);
		int k = map.get(preorder[ps]);
		node.left = buildTree(inorder, is, k - 1, preorder, ps + 1,
				ps + k - is, map);
		node.right = buildTree(inorder, k + 1, ie, preorder, ps + k - is + 1,
				pe, map);
		return node;
	}
}

4.一些总结：