二叉树的建立

整数数组转二叉树

下面只是一个简单的示例，没考虑某个子树为空的情况。把{1, 2, 3, 21, 22, 31, 32} 转变为一个二叉树。

import java.util.ArrayList;
import java.util.List;public class TreeTest {//tree structpublic static 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;}}public static void main(String[] args) {int[] a = {1, 2, 3, 21, 22, 31, 32};//最终循环体执行的是节点，而不是整数值，所以要先把数组转为节点列表，然后遍历List<TreeNode> list = new ArrayList<>();for (int i = 0; i < a.length; i++) {TreeNode t = new TreeNode(a[i]);list.add(t);}for (int i = 0; i < list.size(); i++) {//增加判空，因为可能只有左子树，或者只有右子树if (2 * i + 1 < list.size() && list.get(2 * i + 1) != null) { list.get(i).left = list.get(2 * i + 1);}if (2 * i + 2 < list.size() && list.get(2 * i + 2) != null) {list.get(i).right = list.get(2 * i + 2);}}printNode(list.get(0));}public static void printNode(TreeNode n) { //先序输出if (n != null) {System.out.print(" " + n.val);printNode(n.left);printNode(n.right);}}
}

输出结果：

 1 2 21 22 3 31 32

Object数组转二叉树

要考虑某个节点为空的情况，如下图，数组表示为[1,null,2,null,null,3,null]，就要使用Object数组。

在leetcode94题中，上图用数组表示为[1,null,2,3]，这种表示是不严谨的。不必要纠结这个点，理解根据数组创建二叉树的原理，能自己创建二叉树就好。后面学到二叉树的序列化，再分析。

import java.util.ArrayList;
import java.util.List;public class TreeTest2 {//tree structpublic static 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;}}public static void main(String[] args) {Object[] a = {1, null, 2, null, null, 3, null};List<TreeNode> list = new ArrayList<>();for (int i = 0; i < a.length; i++) {if (a[i] != null) {TreeNode t = new TreeNode((int) a[i]);list.add(t);} else {list.add(null);}}for (int i = 0; i < list.size(); i++) {if (list.get(i) == null) {// i=2continue;}//增加判空，因为可能只有左子树，或者只有右子树if ((2 * i + 1) < list.size() && list.get(2 * i + 1) != null) {list.get(i).left = list.get(2 * i + 1);}if ((2 * i + 2) < list.size() && list.get(2 * i + 2) != null) {list.get(i).right = list.get(2 * i + 2);}}printNode(list.get(0));}public static void printNode(TreeNode n) {/***        1*   null    2*         3   null*/if (n != null) {System.out.print(" " + n.val);printNode(n.left);printNode(n.right);}}
}

输出：

1 2 3

二叉树的遍历

leetcode94.二叉树的中序遍历

题目描述

    public static List<Integer> inorderTraversal(TreeNode root) {List<Integer> list = new ArrayList<>();list = put(root, list);return list;}public static List<Integer> put(TreeNode r, List<Integer> list) {if (r != null) {list = put(r.left, list);list.add(r.val);list = put(r.right, list);}return list;}
}

leetcode144.二叉树的前序遍历

题目描述

class Solution {public List<Integer> preorderTraversal(TreeNode root) {List<Integer> list = new ArrayList<>();addElement(root,list);return list;}public List<Integer> addElement(TreeNode node, List<Integer> list) {if (node != null) {list.add(node.val);addElement(node.left, list);addElement(node.right, list);}return list;}
}