LeetCode 145. 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？

题解:
方法一：递归求解，后续遍历为先左子树，后右子树，最后访问根节点。

/**
 * 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> ans = new ArrayList<Integer>();
        dfs(root,ans);
        return ans;
    }
    public void dfs(TreeNode root,List<Integer>ans){
        if(root == null)
            return;
        dfs(root.left,ans);
        dfs(root.right,ans);
        ans.add(root.val);
    }
}

方法二：非递归求解，迭代求解。显示定义栈，模拟递归过程。

/**
 * 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> ans = new ArrayList<Integer>();
        if(root == null)
            return ans;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode pre = null;
        while(root != null || !stack.isEmpty()){

            while(root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if(root.right == null || root.right == pre){
                ans.add(root.val);
                
                pre = root;
                root = null;
            }
            else{
                stack.push(root);
                root = root.right;
            }
        }
        return ans;
    }
  
}

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

题解参考：https://leetcode-cn.com/problems/binary-tree-postorder-traversal/solution/er-cha-shu-de-hou-xu-bian-li-by-leetcode-solution/