LeetCode145—二叉树的后序遍历（java版）

题目描述：

标签：栈  树 

给定一个二叉树，返回它的 后序 遍历。

 代码：

思路分析：后序遍历是左子树-右子树-根。思路同二叉树的中序遍历。

《方法一：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> list = new ArrayList<Integer>();
        postorder(root,list);
        return list;
    }

    public void postorder(TreeNode root,List<Integer> res){
        if(root == null){
            return;
        }
        postorder(root.left,res);
        postorder(root.right,res);
        res.add(root.val);
    }
}

《方法二：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<Integer>();
        if(root == null){
            return list;
        }
        Deque<TreeNode> stack = new LinkedList<TreeNode>();
        TreeNode prev = null;
        while(root != null || !stack.isEmpty()){
            while(root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if(root.right == null || root.right == prev){
                list.add(root.val);
                prev = root;
                root = null;
            }else{
                stack.push(root);
                root = root.right;
            }
        }
        return list;
    }
}