先自定义一下二叉树的类:
java
// 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;
}
}
一个代码里面同时实现二叉树的前序、中序、后序遍历:
以该二叉树为例
java
import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;
class preorderTraversalSolution {
public static void main(String[] args) {
TreeNode root = new TreeNode(1);
root.left = new TreeNode(2);
root.right = new TreeNode(3);
root.left.left = new TreeNode(4);
root.left.right = new TreeNode(5);
root.right.right = new TreeNode(6);
Solution sol = new Solution();
// 前序、中序、后序
System.out.println(sol.preorderTraversal(root).toString()); //[1, 2, 4, 5, 3, 6]
System.out.println(sol.inorderTraversal(root).toString()); //[4, 2, 5, 1, 3, 6]
System.out.println(sol.postorderTraversal(root).toString()); //[4, 5, 2, 6, 3, 1]
}
}
class Solution {
//前序遍历
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<Integer>();
if (root == null) {
return res;
}
Deque<TreeNode> stack = new LinkedList<TreeNode>();
TreeNode node = root;
while(!stack.isEmpty() || node != null){
while (node != null) {
res.add(node.val); //相比中序遍历,只有这行代码换了位置
stack.push(node);
node = node.left;
}
node = stack.pop();
node = node.right;
}
return res;
}
//中序遍历
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<Integer>();
if (root == null) {
return res;
}
Deque<TreeNode> stack = new LinkedList<TreeNode>();
TreeNode node = root;
while(!stack.isEmpty() || node != null){
while (node != null) {
stack.push(node);
node = node.left;
}
node = stack.pop();
res.add(node.val); //相比前序遍历,只有这行代码换了位置
node = node.right;
}
return res;
}
//后序遍历
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<Integer>();
if (root == null) {
return res;
}
Deque<TreeNode> stack = new LinkedList<TreeNode>();
TreeNode node = root;
TreeNode pre = null;
while(!stack.isEmpty() || node != null){
while (node != null) {
stack.push(node);
node = node.left;
}
node = stack.pop();
if (node.right == null || node.right == pre) {
//如果不存在右子树,则输出该节点的值
//如果该节点的右结点刚刚遍历过了,则也应该输出该节点的值
res.add(node.val);
pre = node;
node = null;
}else{
//如果存在右子树,则重新放回栈中,因为它的值要在右子树遍历完之后添加
stack.push(node);
node = node.right;
}
}
return res;
}
}
三种遍历方式的Java递归实现在这里:算法:Java构建二叉树并递归实现二叉树的前序、中序、后序遍历-CSDN博客