Traversal introduction
1. First-order traversal
Recursion:
class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> list = new ArrayList<>();
if(root == null){
return list;
}
help(root,list);
return list;
}
public void help(TreeNode root,List<Integer> list){
if(root == null){
return ;
}
list.add(root.val);
help(root.left,list);
help(root.right,list);
}
}
Iteration:
public List<Integer> preOrderIteration(TreeNode head) {
if (head == null) {
return;
}
Stack<TreeNode> stack = new Stack<>();
List<Integer> ret = new ArrayList<Integer>();
stack.push(head);
while (!stack.isEmpty()) {
TreeNode node = stack.pop();
ret.add(node.val);
if (node.right != null) {
stack.push(node.right);
}
if (node.left != null) {
stack.push(node.left);
}
}
return ret;
}
2. In-order traversal
The way the stack is implemented:
public class Solution {
public List < Integer > inorderTraversal(TreeNode root) {
List < Integer > res = new ArrayList <> ();
Stack < TreeNode > stack = new Stack <> ();
TreeNode curr = root;
while (curr != null || !stack.isEmpty()) {
while (curr != null) {
stack.push(curr);
curr = curr.left;
}
curr = stack.pop();
res.add(curr.val);
curr = curr.right;
}
return res;
}
}
Recursive way:
class Solution {
private List<Integer> list = new ArrayList<>();
public List<Integer> inorderTraversal(TreeNode root) {
if(root != null){
inorderTraversal(root.left);
list.add(root.val);
inorderTraversal(root.right);
}
return list;
}
}
public List<Integer> help(TreeNode root,List<Integer> list){
if(root == null){
return list;
}
list = help(root.left,list);
list.add(root.val);
list = help(root.right,list);
return list;
}
3. Post-order traversal
Recursion:
class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> list = new ArrayList<>();
if(root == null){
return list;
}
help(root,list);
return list;
}
public void help(TreeNode root,List<Integer> list){
if(root == null){
return ;
}
help(root.left,list);
help(root.right,list);
list.add(root.val);
}
}
Iteration:
first convert to center right and left, and then use the characteristics of the stack to convert to left and right center
public static void postOrderIteration(TreeNode head) {
if (head == null) {
return;
}
Stack<TreeNode> stack1 = new Stack<>();
Stack<TreeNode> stack2 = new Stack<>();
stack1.push(head);
while (!stack1.isEmpty()) {
TreeNode node = stack1.pop();
stack2.push(node);
if (node.left != null) {
stack1.push(node.left);
}
if (node.right != null) {
stack1.push(node.right);
}
}
while (!stack2.isEmpty()) {
System.out.print(stack2.pop().value + " ");
}
}
Direct post-order traversal
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> ans = new ArrayList<Integer>();
Stack<TreeNode> stack = new Stack<TreeNode>();
TreeNode current = root, r = null; //r结点用来区分之前的结点是否被访问过
while(current != null || !stack.isEmpty()){
while(current != null){
stack.push(current);
current = current.left;
}
current = stack.peek(); //看最左结点有没有右子树
if(current.right != null && current.right != r){
current = current.right;
}else{
current = stack.pop(); //访问该结点，并标记被访问
ans.add(current.val);
r = current;
current = null;
}
}
return ans;
}