144. Binary Tree Preorder Traversal

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given a binary tree, return the preorder traversal of its nodes‘ values.

 

Solution 1://非递归

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {  
        List<Integer> result = new ArrayList<>();    
        if (root == null) {  
            return result;  
        }    
        Stack<TreeNode> stack = new Stack<>();  
        stack.push(root);    
        while (!stack.empty()) {   
            TreeNode node = stack.pop();  
            result.add(node.val);    
            if (node.right != null) {  
                stack.push(node.right);  
            }    
            if (node.left != null) {  
                stack.push(node.left);  
            }  
        }    
        return result;  
    }  
}
 
Solution 2://递归
public class Solution {
    List<Integer> result = new ArrayList<>();
    
    public List<Integer> preorderTraversal(TreeNode root) {
        if (root != null)
            helper(root);                    
        return result;
    }
    
    private void helper(TreeNode root) {
        result.add(root.val);
        if (root.left != null)
            helper(root.left);
        if (root.right != null)
            helper(root.right);
    }
}

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