Binary Tree
Pre-order traversal
parent, right, left in each level
public class Solution {
public ArrayList<Integer> preorderTraversal(TreeNode root) {
ArrayList<Integer> res = new ArrayList<>();
if (root == null) {
return res;
}
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
while (!stack.empty()) {
TreeNode cur = stack.pop();
res.add(cur.val);
if (cur.right != null) {
stack.push(cur.right);
}
if (cur.left != null) {
stack.push(cur.left);
}
}
return res;
}
}
In-order traversal
left, parent, right in each level
public class Solution {
public ArrayList<Integer> inorderTraversal(TreeNode root) {
ArrayList<Integer> res = new ArrayList<>();
if (root == null) {
return res;
}
Stack<TreeNode> stack = new Stack<>();
TreeNode cur = root;
while (cur != null || !stack.empty()) {
while (cur != null) {
stack.push(cur);
cur = cur.left;
}
cur = stack.pop();
res.add(cur.val);
cur = cur.right;
}
return res;
}
}
Post-order traversal
right, parent, left in each level
Binary Tree Level Order Traversal
[ [root], [root.left, root.right], ... ]
public class Solution {
public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
ArrayList<ArrayList<Integer>> res = new ArrayList<>();
if (root == null) {
return res;
}
Queue<TreeNode> q = new LinkedList<>();
q.offer(root);
while (!q.isEmpty()) {
int size = q.size();
ArrayList<Integer> list = new ArrayList<>();
for (int i = 0; i < size; i++) {
TreeNode node = q.poll();
list.add(node.val);
if (node.left != null) {
q.offer(node.left);
}
if (node.right != null) {
q.offer(node.right);
}
}
res.add(list);
}
return res;
}
}