• [leetcode] Binary Tree Postorder Traversal


    题目:(Tree ,Stack)

    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].

    题解:


    binary tree 三部曲里面最难的一个。多看看。

    /**
     * Definition for binary tree
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    public class Solution {
        public ArrayList<Integer> postorderTraversal(TreeNode root) {
           ArrayList<Integer> result = new ArrayList<Integer>();
           if(root==null)
                return result;
           
           Stack<TreeNode> stack=new Stack<TreeNode>();
           stack.push(root);
           TreeNode prev=null;
           
           while(!stack.isEmpty()){
               TreeNode current=stack.peek();
               if(prev==null||prev.left==current||prev.right==current)
               {
                   if(current.left!=null)
                       stack.push(current.left);
                   else if(current.right!=null)
                       stack.push(current.right);
                   else
                   {
                       stack.pop();
                       result.add(current.val);
                   }
               }else if(prev==current.left)
               {
                   if(current.right!=null)
                       stack.push(current.right); 
                   else
                   {
                       stack.pop();
                       result.add(current.val);
                   }
               }else
               {
                   stack.pop();
                   result.add(current.val);
               }
               prev=current;
           }
           return result;
        }
    }
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  • 原文地址:https://www.cnblogs.com/fengmangZoo/p/4196980.html
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