94. Binary Tree Inorder Traversal

JIAKAOBO

LeetCode

感谢赞助!

• ㊗️
• 大家
• offer
• 多多！

Problem

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]

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

Code

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

    private void helper(TreeNode root){
        if(root == null) return;

        helper(root.left);
        res.add(root.val);
        helper(root.right);
    }
}

/**
 * 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<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode curr = root;

        while(curr != null || !stack.isEmpty()){
            while(curr != null){
                stack.push(curr);
                curr = curr.left;
            }

            curr = stack.pop();
            res.add(curr.val);

            curr = curr.right;
        }

        return res;
    }
}