94.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

// recursion
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);
            }
        }
    }
}

// stack
// 指针curr
// while左边一直有值或curr指向一个non-null node，stack push 每一个值， 左边到底后，指针从stack顶端开始pop， pop完之后res加入current值，指针往
// current.right指
public 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;
    }
}