leetcode 94 binary tree inorder traversal

leetcode 94 binary tree inorder 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]

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solution one

Recursive Approach

/**
 * 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 > res = new ArrayList < > ();
        helper(root, res);
        return res;
    }

    public void helper(TreeNode root, List < Integer > res) {
        if (root != null) {
            if (root.left != null) {
                helper(root.left, res);
            }
            res.add(root.val);
            if (root.right != null) {
                helper(root.right, res);
            }
        }
    }
}

---

solution two

iterating method using stack

class Solution {
    public List<Integer> inorderTrversal(TreeNode root){
        List<Integer> list = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode cur = root;
        while(cur!=null || !stack.empty()) {
            while(cur!=null){
                stack.push(cur);
                cur = cur.left;
            }
            cur = stack.pop();
            list.add(cur.val);
            cur = cur.right;
        }
        return list;
    } 
}