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

栈（复习）

/*** 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> res = new ArrayList<Integer>();Deque<TreeNode> stack = new LinkedList<TreeNode>();// 新增pre记录TreeNode preAccess = null;while(root!=null||!stack.isEmpty()){while(root!=null){stack.push(root);root=root.left;}root=stack.pop();if(root.right==null||root.right==preAccess){res.add(root.val);preAccess=root;root=null;}else{stack.push(root);root=root.right;}}return res;}
}