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;
}
}
