每日一题——二叉树的中序遍历

示例:
输入: [1,null,2,3]
   1
    \
     2
    
   3


输出: [1,3,2]

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

// 递归法
class Solution {


    private List<Integer> result = new ArrayList<>();


    public List<Integer> inorderTraversal(TreeNode root) {
        if (root == null) return result;
        inorderTraversal(root.left);
        result.add(root.val);
        inorderTraversal(root.right);
        return result;
    }
    
}

// 迭代法
class Solution {


    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) return result;
        Stack<TreeNode> stack = new Stack<>();
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            result.add(root.val);
            root = root.right;
        }
        return result;
    }


}