【简单】Lintcode 66:Binary Tree Preorder Traversal

最新推荐文章于 2021-10-25 17:39:07 发布

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最新推荐文章于 2021-10-25 17:39:07 发布

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本文介绍了一种实现二叉树前序遍历的方法，包括递归和非递归两种方式。递归方法通过直接调用自身实现节点的访问，而非递归方法则借助栈来保存待访问的节点。

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

Example

Given:

return [1,2,4,5,3].

解题思路1：递归。

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */




class Solution {
public:
    /*
     * @param root: A Tree
     * @return: Preorder in ArrayList which contains node values.
     */
     vector<int> result;
    vector<int> preorderTraversal(TreeNode * root)
    {
        // write your code here
        if(root == NULL) return result;
        
        result.push_back(root->val);
        
        if(root->left != NULL)
            result = preorderTraversal(root->left);
            
        if(root->right != NULL)
            result = preorderTraversal(root->right);
            
        return result;
    }
};

JAVA代码：递归：

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: A Tree
     * @return: Preorder in ArrayList which contains node values.
     */
    public List<Integer> preorderTraversal(TreeNode root) {
        // write your code here
        List<Integer> list = new ArrayList<>();
        
        preorderTraversal(root , list);
        
        return list;
    }
    
    public void preorderTraversal(TreeNode root , List<Integer> list){
        if(root == null)
            return;
        
        list.add(root.val);
        preorderTraversal(root.left , list);
        preorderTraversal(root.right , list);
    }
}

JAVA代码：非递归，利用栈：

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: A Tree
     * @return: Preorder in ArrayList which contains node values.
     */
    public List<Integer> preorderTraversal(TreeNode root) {
        // write your code here
        List<Integer> list = new ArrayList<>();
        if(root == null)
            return list;
            
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        
        while(!stack.isEmpty()){
            TreeNode node = stack.pop();
            list.add(node.val);
            
            //注意这里先压右栈，再压左栈，与前序遍历相反
            if(node.right != null)
                stack.push(node.right);
            if(node.left != null)
                stack.push(node.left);
        }
        
        
        return list;
    }
    
}