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PreorderTraversal.java
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PreorderTraversal.java
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package trees.easy;
import trees.TreeNode;
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;
/***
* Problem 144 in Leetcode: https://leetcode.com/problems/binary-tree-preorder-traversal/
*
* Given the root of a binary tree, return the preorder traversal of its nodes' values.
*
* Example 1:
* Input: root = [1,null,2,3]
* Output: [1,2,3]
*
* Example 2:
* Input: root = [1]
* Output: [1]
*
* Example 3:
* Input: root = []
* Output: []
*/
public class PreorderTraversal {
public static void main(String[] args) {
TreeNode root = new TreeNode(1);
root.right = new TreeNode(2);
root.right.left = new TreeNode(3);
ArrayList<Integer> list = new ArrayList<>();
System.out.println("Using Recursion: " + preorderUsingRecursion(root, list));
System.out.println("Using Stack: " + preorderUsingStack(root));
}
private static List<Integer> preorderUsingRecursion(TreeNode root, ArrayList<Integer> list) {
if (root != null) {
list.add(root.value);
preorderUsingRecursion(root.left, list);
preorderUsingRecursion(root.right, list);
}
return list;
}
private static List<Integer> preorderUsingStack(TreeNode root) {
ArrayList<Integer> list = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
while (root != null || !stack.isEmpty()) {
while (root != null) {
list.add(root.value);
stack.push(root);
root = root.left;
}
root = stack.pop();
root = root.right;
}
return list;
}
}