p106

LeetCode p106 Construct Binary Tree from Inorder and Postorder Traversal 题解

1.题目：

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

题意：

输入一二叉树的中序遍历，后续遍历，返回这个二叉树。

2.解题思路:

后续的最后一个为头，切分中序，递归。

3.代码

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[title] [] [url] [link text]

 
/**
 * 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[] inorder, int[] postorder) {
        if (inorder.length == 0)
			return null;
		int len = postorder.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, postorder, 0, len - 1, map);
	}

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

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4.一些总结：