preorder, inorder, postorder traversal

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本文详细介绍了如何使用递归和迭代方法实现二叉树的前序遍历，并提供了对应的Java代码实现，旨在提高对二叉树遍历算法的理解和实践能力。

preorder, inorder, postorder traversal

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

For example:
Given binary tree {1,#,2,3},

return [1,2,3].

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

Recursive solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<Integer>();
        helper(root, ret);
        return ret;
    }
    
    private void helper(TreeNode root, List<Integer> ret) {
        if (root == null)
            return;
        
        ret.add(root.val);
        helper(root.left, ret);
        helper(root.right, ret);
    }
}

Iterative solution

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<Integer>();
        if (root == null)
            return ret;
            
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        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;
        
        
    }
    
    
}