_144.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.TreeNode;

import java.util.*;

/**
 * 144. Binary Tree Preorder Traversal
 * Given a binary tree, return the preorder traversal of its nodes' values.

 For example:
 Given binary tree {1,#,2,3},
 1
  \
   2
  /
 3
 return [1,2,3].

 Note: Recursive solution is trivial, could you do it iteratively?*/

public class _144 {

    public List<Integer> preorderTraversal_iterative(TreeNode root) {
        List<Integer> list = new ArrayList();
        if(root == null) return list;
        Deque<TreeNode> stack = new ArrayDeque<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode curr = stack.pop();
            list.add(curr.val);
            /**We push right nodes onto the stack first, since they'll be popped out later than
             * the left nodes, to meet the preorder: root -> left -> right. */
            if(curr.right != null) stack.push(curr.right);
            if(curr.left != null) stack.push(curr.left);
        }
        return list;
    }

    public List<Integer> preorderTraversal_recursive(TreeNode root) {
        List<Integer> list = new ArrayList();
        return pre(root, list);
    }
    
    List<Integer> pre(TreeNode root, List<Integer> list){
        if(root == null) return list;
        list.add(root.val);
        pre(root.left, list);
        pre(root.right, list);
        return list;
    }

}