94.binary-tree-inorder-traversal.java

import java.util.ArrayList;

/*
 * @lc app=leetcode id=94 lang=java
 *
 * [94] Binary Tree Inorder Traversal
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Stack<TreeNode> stk = new Stack<>();
        TreeNode cur = root;
        while (cur!= null || !stk.isEmpty()) { 
            while (cur != null) {
                stk.push(cur);
                cur = cur.left;
            }
            cur = stk.pop();
            res.add(cur.val);
            cur = cur.right;
        }
        return res;
    }
}
// @lc code=end

/* class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        inorder(root, res);
        return res;
    }

    void inorder(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }
        inorder(root.left, list);
        list.add(root.val);
        inorder(root.right, list);
    }
} */