Binary Tree Preorder Traversal

最新推荐文章于 2022-11-29 02:00:43 发布

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本文介绍了一种给定二叉树返回节点值的前序遍历方法，包括递归实现与迭代实现两种方式。递归实现简洁直观，而迭代实现使用了栈来保存节点，确保了遍历顺序正确。

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Description: 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?

1. Recursive implementation:

Update 13/01/2014:

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

old version:

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> preorder = new ArrayList<Integer>();
        
        if(root==null) return preorder;
        preorder.add(root.val);
        if(root.left!=null) preorder.addAll(preorderTraversal(root.left));
        if(root.right!=null) preorder.addAll(preorderTraversal(root.right));
        return preorder;
    }
}

2. Iterative implementation

1) Create an empty stack nodeStack and push root node to stack.
2) Do following while nodeStack is not empty.
….a) Pop an item from stack and print it.
….b) Push right child of popped item to stack

….c) Push left child of popped item to stack

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> preorder = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode current = null;
        
        // empty tree
        if(root==null) return preorder;
        stack.push(root);
        
        while(!stack.empty()) {
            current = stack.pop();
            preorder.add(current.val);
            if(current.right!=null) stack.push(current.right);         // push right subtree into stack
            if(current.left!=null) stack.push(current.left);          // push left subtree into stack
        }
        
        return preorder;
    }
}