94. 二叉树的中序遍历

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
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> lists = new ArrayList<>();
        inorderTraversalHelper(root,lists);
        return lists;
    }
    private void inorderTraversalHelper(TreeNode node,List<Integer> res){
        if (node != null){
            if (node.left!=null){
                inorderTraversalHelper(node.left,res);
            }
            res.add(node.val);
            if (node.right!=null){
                inorderTraversalHelper(node.right,res);
            }
        }
    }
}
//非递归 利用栈实现
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> lists = 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();
            lists.add(curr.val);
            curr = curr.right;
        }
        return lists;
    }
}