[Leetcode] Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes' values.

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

   1
    
     2
    /
   3

return [1,3,2].

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

Solution:

1. 非递归

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result=new ArrayList<Integer>();
        if(root==null)
            return result;
        Stack<TreeNode> s=new Stack<TreeNode>();
        s.add(root);
        TreeNode p=root.left;
        while(!s.isEmpty()){
            while(p!=null){
                s.add(p);
                p=p.left;
            }
            TreeNode n=s.pop();
            result.add(n.val);
            p=n.right;
            if(p!=null){
                s.add(p);
                p=p.left;
            }
        }
        
        return result;
    }
}

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    ArrayList<Integer> result = new ArrayList<Integer>();
    
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        

    Stack<TreeNode> stack = new Stack<TreeNode>();
        while (root != null || !stack.empty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            if (stack.size() > 0) {
                root = stack.pop();
                result.add(root.val);
                root = root.right;
            }
        }

        return result;
        
    }
}

2. 递归

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result=new ArrayList<Integer>();    
        myInorderTraversal(root,result);  
        return result;
    }

    private void myInorderTraversal(TreeNode root, List<Integer> result) {
        // TODO Auto-generated method stub
        if(root!=null){
            myInorderTraversal(root.left, result);
            result.add(root.val);
            myInorderTraversal(root.right, result);
        }
    }
}