Binary tree traversal [learning algorithm]
Preface
2023-8-7 15:13:50
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recommend
Chapter 6 Tree [Data Structure and Algorithm]
Binary tree traversal [learning algorithm]
preorder traversal
144. Preorder traversal of binary tree
Recursive implementation
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
ArrayList<Integer> res=new ArrayList<>();
preorderTraversal(root,res);
return res;
}
public void preorderTraversal(TreeNode root,ArrayList<Integer> list) {
if(root!=null){
list.add(root.val);
preorderTraversal(root.left,list);
preorderTraversal(root.right,list);
}
}
}
Recursive non-implementation -1
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
ArrayList<Integer> res=new ArrayList<>();
preorderTraversal(root,res);
return res;
}
public void preorderTraversal(TreeNode root,ArrayList<Integer> list) {
Stack<TreeNode> stack=new Stack<>();
TreeNode cur=root;
while(cur!=null||!stack.isEmpty()){
while(cur!=null){
list.add(cur.val);
stack.push(cur);
cur=cur.left;
}
if(!stack.isEmpty()){
cur=stack.pop();
cur=cur.right;
}
}
}
}
Recursive non-implementation-2
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
ArrayList<Integer> res=new ArrayList<>();
preorderTraversal(root,res);
return res;
}
public void preorderTraversal(TreeNode root,ArrayList<Integer> list) {
Stack<TreeNode> stack=new Stack<>();
TreeNode cur=root;
while(cur!=null||!stack.isEmpty()){
if(cur!=null){
list.add(cur.val);
stack.push(cur);
cur=cur.left;
}else{
cur=stack.pop();
cur=cur.right;
}
}
}
}
inorder traversal
94. In-order traversal of binary tree
Recursive implementation
class Solution {
//递归
public List<Integer> inorderTraversal(TreeNode root) {
ArrayList<Integer> res=new ArrayList<>();
inorderTraversal(root,res);
return res;
}
public void inorderTraversal(TreeNode root,ArrayList<Integer> list) {
if(root!=null){
inorderTraversal(root.left,list);
list.add(root.val);
inorderTraversal(root.right,list);
}
}
}
Non-recursive implementation-1
class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> list=new ArrayList<>();
Stack<TreeNode> stack=new Stack();
TreeNode cur=root;
while(cur!=null||!stack.isEmpty()){
//当前结点指针及栈均空,则结束
while(cur!=null){
//访问根结点,根指针进栈,进入左子树
stack.push(cur);
cur=cur.left;
}
if(!stack.isEmpty()){
cur=stack.pop();
list.add(cur.val);
cur=cur.right;//根指针退栈,进入其右子树
}
}
return list;
}
}
Non-recursive implementation-2
class Solution {
//递归
public List<Integer> inorderTraversal(TreeNode root) {
ArrayList<Integer> res=new ArrayList<>();
inorderTraversal(root,res);
return res;
}
public void inorderTraversal(TreeNode root,ArrayList<Integer> list) {
Stack<TreeNode> stack=new Stack<>();
TreeNode cur=root;
//结点不为空或栈不为空
while(cur!=null||!stack.isEmpty()){
//结点不为空
if(cur!=null){
stack.push(cur);
cur=cur.left;
}else{
cur=stack.pop();
list.add(cur.val);
cur=cur.right;
}
}
}
}
Postorder traversal
recursion
class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
ArrayList<Integer> res=new ArrayList<>();
postorderTraversal(root,res);
return res;
}
public void postorderTraversal(TreeNode root,ArrayList<Integer> list) {
if(root!=null){
postorderTraversal(root.left,list);
postorderTraversal(root.right,list);
list.add(root.val);
}
}
}
non-recursive
class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
ArrayList<Integer> res=new ArrayList<>();
Stack<TreeNode> stack=new Stack<>();
TreeNode cur=root,next=null;
while(cur!=null||!stack.isEmpty()){
//先把结点给全进栈
while(cur!=null){
stack.push(cur);
cur=cur.left;//左
}
//栈不为空
if(!stack.isEmpty()){
cur=stack.peek();
if(cur.right==null||cur.right==next){
//判断栈顶结点的右子树是否为空，或者刚被访问过
cur=stack.pop();
res.add(cur.val);//根
next=cur;//记录刚访问
cur=null;//清空
}else{
cur=cur.right;//右
}
}
}
return res;
}
}
at last
2023-8-7 16:00:45
We all have a bright future
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