145. Binary Tree Postorder Traversal

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/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } *//** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public List<Integer> postorderTraversal(TreeNode root) {        List<Integer> ls = new ArrayList<Integer>();        if(root==null)              return ls;         Stack<TreeNode> st = new Stack<TreeNode>();          HashSet<TreeNode> hs = new HashSet<TreeNode>();          st.push(root);          while(!st.isEmpty())         {             TreeNode temp = st.pop();             if(hs.contains(temp))             {                 ls.add(temp.val);                 continue;             }             hs.add(temp);             st.push(temp);             if(temp.right!=null)                 st.push(temp.right);             if(temp.left!=null)                 st.push(temp.left);         }         return ls;       }}

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