Leetcode-Binary Tree Postorder Traversal

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Given a binary tree, return the postorder traversal of its nodes‘ values.

For example:
Given binary tree {1,#,2,3},

   1         2    /   3

return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

Solution:

 1 /** 2  * Definition for binary tree 3  * public class TreeNode { 4  *     int val; 5  *     TreeNode left; 6  *     TreeNode right; 7  *     TreeNode(int x) { val = x; } 8  * } 9  */10 public class Solution {11     public List<Integer> postorderTraversal(TreeNode root) {12         List<Integer> res = new ArrayList<Integer>();13         if (root==null) return res;14 15         Stack<TreeNode> nStack = new Stack<TreeNode>();16         Stack<Integer> record = new Stack<Integer>();17         nStack.push(root);18         record.push(0);19 20         while (nStack.size()!=0){21             TreeNode cur = nStack.peek();22             if (cur.left==null && cur.right==null){23                 res.add(cur.val);24                 nStack.pop();25                 record.pop();26                 continue;27             }28 29             int val = record.peek();30             if (val==0){31                 record.pop();32                 record.push(val+1);33                 if (cur.left!=null){34                     nStack.push(cur.left);35                     record.push(0);36                 }37                 continue;38             }39 40             if (val==1){41                 record.pop();42                 record.push(val+1);43                 if (cur.right!=null){44                     nStack.push(cur.right);45                     record.push(0);46                 }47                 continue;48             }49 50             if (val==2){51                 res.add(cur.val);52                 nStack.pop();53                 record.pop();54                 continue;55             }56         }57 58         return res;  59         60     }61 }

 

Leetcode-Binary Tree Postorder Traversal