https://www.b2bchain.cn/3761.html
原文地址 https://www.b2bchain.cn/3761.html
144.前序
/**
* 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> preorderTraversal(TreeNode root) {
List<Integer> ans=new ArrayList<>();
helper(root,ans);
return ans;
}
private void helper(TreeNode root,List ans){
if(root==null) return ;
ans.add(root.val);
helper(root.left,ans);
helper(root.right,ans);
}
}
/**
* 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> preorderTraversal(TreeNode root) {
List<Integer> ans=new ArrayList<>();
if(root==null) return ans;
Deque<TreeNode> stack=new ArrayDeque<>();
stack.push(root);
while(!stack.isEmpty()){
TreeNode cur=stack.pop();
ans.add(cur.val);
if(cur.right!=null){
stack.push(cur.right);
}
if(cur.left!=null){
stack.push(cur.left);
}
}
return ans;
}
}
中序94
/**
* 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) {
List<Integer> ans=new ArrayList<>();
if(root==null) return ans;
Deque<TreeNode> stack=new ArrayDeque<>();
while(!stack.isEmpty()||root!=null){
//入棧一直到最左
while(root!=null){
stack.push(root);
root=root.left;
}
root=stack.pop();
ans.add(root.val);
root=root.right;
}
return ans;
}
}
後序145
/**
* 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> postorderTraversal(TreeNode root) {
//linkedList 涉及到結果的反轉
LinkedList<Integer> ans=new LinkedList<>();
if(root==null) return ans;
Deque<TreeNode> stack=new ArrayDeque<>();
stack.push(root);
while(!stack.isEmpty()){
TreeNode cur=stack.pop();
//加到首
ans.addFirst(cur.val);
//使用if
if(cur.left!=null){
stack.push(cur.left);
}
if(cur.right!=null){
stack.push(cur.right);
}
}
return ans;
}
}