[leetCode]145. 二叉树的后序遍历

题目

链接：https://leetcode-cn.com/problems/binary-tree-postorder-traversal

给定一个二叉树，返回它的 后序 遍历。

示例:

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 

输出: [3,2,1]

递归

class Solution {
    List<Integer> ans = new ArrayList<>();

    public List<Integer> postorderTraversal(TreeNode root) {
        traversal(root);
        return ans;
    }

    private void traversal(TreeNode node) {
        if (node == null)
            return;
        traversal(node.left);
        traversal(node.right);
        ans.add(node.val);
    }
}

迭代

class Solution {
    List<Integer> ans = new ArrayList<>();

    public List<Integer> postorderTraversal(TreeNode root) {
        if (root == null) return ans;
        LinkedList<TreeNode> stack = new LinkedList<>();
        stack.push(root);
        while(!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node != null) {
                stack.push(node);
                stack.push(null);
                if (node.right != null) 
                    stack.push(node.right);
                if (node.left != null)
                    stack.push(node.left);
            } else {
                TreeNode cur = stack.pop();
                ans.add(cur.val);
            }
        }
        return ans;
    }
}