LeetCode 94. Binary Tree Inorder Traversal

94. Binary Tree Inorder Traversal

Difficulty:: Medium

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?

Solution

Language: Java

/**
 * 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> result = new ArrayList<>();
            traverse(root, result);
            return result;
     }
      
    // 递归版本
      private void traverse(TreeNode root, List<Integer> result) {
            if (root == null) {
                  return;
           }
            traverse(root.left, result);
            result.add(root.val);
            traverse(root.right, result);
     }
    
    // 非递归版本
    private void traverseIterative(TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }
        Deque<TreeNode> stack = new ArrayDeque<>();
        while (root != null || !stack.isEmpty()) {
            if (root != null) {
                stack.push(root);
                root = root.left;
            } else {
                root = stack.pop();
                result.add(root.val);
                root = root.right;
            }
        }
    }
}