145.BinaryTreePostorderTraversal.java

/**
 * 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; } }
 */

// 解法一：递归
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        postorder(root, res);
        return res;
    }

    public void postorder(TreeNode node, List<Integer> res) {
        if (node == null)
            return;

        postorder(node.left, res);
        postorder(node.right, res);
        res.add(node.val);
    }
}

// 解法二：迭代
// 没做完

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList();

        if (root != null)
            return res;

        Deque<TreeNode> stack = new LinkedList();
        TreeNode node = root;

        while (!stack.isEmpty() || node != null) {
            while (node != null) {
                stack.push(node);
                node = node.left;
            }

        }

    }
}