leetcode145. 二叉树的后序遍历

思路一:

递归写法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer>list1 = new LinkedList<Integer>();
    public List<Integer> postorderTraversal(TreeNode root) {
        if(root==null){
            return list1;
        }
        postorderTraversal(root.left);
        postorderTraversal(root.right);
        list1.add(root.val);
        return list1;
    }
}

思路二：

迭代写法  

运用两个栈还原递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer>list1 = new LinkedList<Integer>();
    public List<Integer> postorderTraversal(TreeNode root) {
        Stack<TreeNode>stk1 = new Stack<TreeNode>();
        Stack<TreeNode>stk2 = new Stack<TreeNode>();
        if(root==null){
            return list1;
        }
        stk1.push(root);
        while(!stk1.empty()){
            stk2.push(stk1.pop());
            if(stk2.peek().left!=null){
                stk1.push(stk2.peek().left);
            }
            if(stk2.peek().right!=null){
                stk1.push(stk2.peek().right);
            }
        }
        while(!stk2.empty()){
            list1.add(stk2.pop().val);
        }
        return list1;
    }
}