94.binary-tree-inorder-traversal.java

/*
 * @lc app=leetcode id=94 lang=java
 *
 * [94] Binary Tree Inorder Traversal
 *
 * https://leetcode.com/problems/binary-tree-inorder-traversal/description/
 *
 * algorithms
 * Medium (55.10%)
 * Total Accepted:    426.4K
 * Total Submissions: 767.5K
 * Testcase Example:  '[1,null,2,3]'
 *
 * 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?
 *
 */
/**
 * 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> list = new ArrayList<>();
    Stack<TreeNode> stack = new Stack<>();
    TreeNode p = root;

    while (!stack.empty() || p != null) {
      while (p != null) {
        stack.push(p);
        p = p.left;
      }
      p = stack.pop();
      list.add(p.val);
      p = p.right;
    }

    return list;
  }
}