前言
二叉树有三种遍历方式
前序遍历:根,左,右
中序遍历:左,根,右
后序遍历:左,右,根
发现规律没?左右的位置始终不变,前序遍历,根在前面,中序遍历,根在中间,以此类推。
前序遍历:A B C D E F
中序遍历:C B D A E F
后序遍历:C D B F E A
前序遍历
LeetCode 144测试通过(LeetCode为一个在线编程网站)
递归实现
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
List<Integer> vals = new ArrayList<>();
public List<Integer> preorderTraversal(TreeNode root) {
if (root != null) {
vals.add(root.val);
if (root.left != null) {
this.preorderTraversal(root.left);
}
if (root.right != null) {
this.preorderTraversal(root.right);
}
}
return vals;
}
}
非递归实现
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
List<Integer> vals = new ArrayList<>();
public List<Integer> preorderTraversal(TreeNode root) {
Stack<TreeNode> nodes = new Stack<>();
List<Integer> vals = new ArrayList<>();
if (root != null)
nodes.push(root);
while (!nodes.isEmpty()) {
//获取头部元素,并且移除
TreeNode treeNode = nodes.pop();
vals.add(treeNode.val);
if (treeNode.right != null) {
nodes.push(treeNode.right);
}
if (treeNode.left != null){
nodes.push(treeNode.left);
}
}
return vals;
}
}
中序遍历
LeetCode 94测试通过
递归实现
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
List<Integer> vals = new ArrayList<>();
public List<Integer> inorderTraversal(TreeNode root) {
if (root != null) {
if (root.left != null) {
this.inorderTraversal(root.left);
}
vals.add(root.val);
if (root.right != null) {
this.inorderTraversal(root.right);
}
}
return vals;
}
}
非递归实现
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
Stack<TreeNode> nodes = new Stack<>();
List<Integer> vals = new ArrayList<>();
while (root != null || !nodes.isEmpty()) {
while (root != null) {
nodes.push(root);
root = root.left;
}
// 返回值同时删除
root = nodes.pop();
vals.add(root.val);
root = root.right;
}
return vals;
}
}
后序遍历
LeetCode 145测试通过
递归实现
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
List<Integer> vals = new ArrayList<>();
public List<Integer> postorderTraversal(TreeNode root) {
if (root != null) {
if (root.left != null) {
this.postorderTraversal(root.left);
}
if (root.right != null) {
this.postorderTraversal(root.right);
}
vals.add(root.val);
}
return vals;
}
}
非递归实现以后会补在原文链接中