leetcode144. 二叉树的前序遍历

1.题目描述：

给你二叉树的根节点root，返回它节点值的前序遍历。

 2.递归前序遍历：

/**
 * 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 {
    private List<Integer> list = new ArrayList<>();
    public List<Integer> preorderTraversal(TreeNode root) {
        preOrder(root);
        return list;
    }
    public void preOrder(TreeNode root) {
        if (root == null) return;
        list.add(root.val);
        preOrder(root.left);
        preOrder(root.right);
    }
}

3.使用栈的迭代前序遍历：

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 temp = stack.pop();
            if (temp == null) continue;
            list.add(temp.val);
            stack.push(temp.right);
            stack.push(temp.left);
        }
        return list;
    }
}