题目描述
思路
- 递归：额外写一个函数void postOrder(TreeNode node, List
res) - 迭代：
- 前序遍历：根---左---右
- 将前序遍历改造成：根---右---左
- 然后反转根右左为：左---右---根，即为后序遍历
- 优化一下：
while (!stack.isEmpty()) {
TreeNode node = stack.pop();
res.addFirst(node.val);
if (node.left != null) stack.push(node.left);
if (node.right != null) stack.push(node.right);
}
方法一：递归
/**
* 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;
* }
* }
*/
class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
postOrder(root, res);
return res;
}
private void postOrder(TreeNode node, List<Integer> res) {
if (node == null) return;
postOrder(node.left, res);
postOrder(node.right, res);
res.add(node.val);
}
}
方法二：迭代
/**
* 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;
* }
* }
*/
class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
Deque<TreeNode> stack = new ArrayDeque<>();
if (root == null) return res;
stack.push(root);
while (!stack.isEmpty()) {
TreeNode node = stack.pop();
res.add(node.val);
if (node.left != null) stack.push(node.left);
if (node.right != null) stack.push(node.right);
}
Collections.reverse(res);
return res;
}
}
优化一下：
/**
* 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;
* }
* }
*/
class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
Deque<TreeNode> stack = new ArrayDeque<>();
if (root == null) return res;
stack.push(root);
while (!stack.isEmpty()) {
TreeNode node = stack.pop();
res.addFirst(node.val);
if (node.left != null) stack.push(node.left);
if (node.right != null) stack.push(node.right);
}
//Collections.reverse(res);
return res;
}
}