Leetcode145. 二叉树的后序遍历

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]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

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> postorderTraversal(TreeNode root) {
        List<Integer>  keys = new ArrayList<>();
        postorderTraversal(root, keys);
        return keys;
    }
    public static void postorderTraversal(TreeNode root, List<Integer> keys){
        if(root == null){
            return;
        }
        //先遍历左子树
        postorderTraversal(root.left, keys);
        //再遍历右子树
         postorderTraversal(root.right, keys);
        //遍历根节点
         keys.add(root.val);
    }
}

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> postorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        //后序遍历：left->right->root  倒过来是root->right->left  则此过程和前序遍历相似
        //前序遍历迭代是：root->left->right
        while(!stack.isEmpty()){
            TreeNode node = stack.pop();
            if(node == null){
                continue;
            }
            list.add(node.val);
            if(node.left!=null){
                stack.push(node.left);
            }
            if(node.right!=null){
                stack.push(node.right);
            }
        }
        Collections.reverse(list);
        //正确的结果需要对list进行翻转
          return list;
    }
}