Binary Tree Inorder Traversal - The Coding Shala

Last Updated: 19-Jan-2021
Home >> Data Structures >> Binary Tree Inorder Traversal

 In this post, we will learn how to Traverse a Binary Tree in In-Order.

Binary Tree In-Order Traversal

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

Example:
Input: [1,null,2,3]
   1
    \
     2
    /
   3
Output: [1,3,2]

In-Order Traversal of Binary Tree in Java

Approach 1

Using Recursion.

Java Program: 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> InOrder = new ArrayList<Integer>();
        if(root == null) return InOrder;
        InOrder.addAll(inorderTraversal(root.left));
        InOrder.add(root.val);
        InOrder.addAll(inorderTraversal(root.right));
        return InOrder;
    }
}

Approach 2

Using Stack[DFS].

Java Program: 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<Integer>();
        if(root == null) return ans;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while(root != null || !stack.empty()){
            while(root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            ans.add(root.val);
            root = root.right;
        }
        return ans;
    }
}


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