Leetcode144. 二叉树的前序遍历

给你二叉树的根节点 root ，返回它节点值的 前序 遍历。

递归：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> keys = new ArrayList<>();
        preorderTraversal(root, keys);
        return keys;
    }
    public void preorderTraversal(TreeNode root, List<Integer> keys){
          if(root == null){
            return;
        }
            //先遍历根节点
             keys.add(root.val);
             //遍历左子树
             preorderTraversal(root.left, keys);
             //遍历右子树
             preorderTraversal(root.right, keys);
    }
}

迭代:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode node = stack.pop();
            if(node == null){
                continue;
            }
            list.add(node.val);
            //前序 遍历，栈的话，需要push先右后左
            if(node.right!=null){
                stack.push(node.right);
            }
            if(node.left!=null){
                 stack.push(node.left);
            }  
        }
        return list;
    }
}