Binary Tree Preorder Traversal

39% Accepted

Given a binary tree, return the preorder traversal of its nodes' values.

Have you met this question in a real interview? Yes
Example
Given binary tree {1,#,2,3}:

1
 \
  2
 /
3
return [1,2,3].

Challenge

Can you do it without recursion?

Tags Expand

• Recursion
• Binary Tree
• Binary Tree Traversal
• Non Recursion

非递归

public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Inorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        // write your code here
        if(root == null){
            return new ArrayList<Integer>();
        }

        ArrayList<Integer> result = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();

        TreeNode cur = root;
        while(!stack.isEmpty() || cur != null){
            while(cur != null){
                stack.push(cur);
                result.add(cur.val);
                cur = cur.left;
            }

            cur = stack.peek();
            stack.pop();
            cur = cur.right;
        }

        return result;
    }
}

递归一(ArrayList声明在函数里)

public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<Integer>();
        // null or leaf
        if (root == null) {
            return result;
        }

        // Divide
        ArrayList<Integer> left = preorderTraversal(root.left);
        ArrayList<Integer> right = preorderTraversal(root.right);

        // Conquer
        result.add(root.val);
        result.addAll(left);
        result.addAll(right);
        return result;
    }
}

递归二(ArrayList声明在全局)

public class Solution {

    ArrayList<Integer> result = new ArrayList<Integer>();

    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        // null or leaf
        if (root == null) {
            return result;
        }
        result.add(root.val);
        ArrayList<Integer> left = preorderTraversal(root.left);
        ArrayList<Integer> right = preorderTraversal(root.right);
        return result;
    }
}