给定一个二叉树,返回它的中序 遍历。
示例:
输入: [1,null,2,3]
1
\
2
/
3
输出: [1,3,2]
进阶: 递归算法很简单,你可以通过迭代算法完成吗?
解题思路:
- 递归
Java代码:
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> res = new LinkedList<>();
recursion(res, root);
return res;
}
private void recursion(List<Integer> res, TreeNode root){
if(null == root)
return;
recursion(res, root.left);
res.add(root.val);
recursion(res, root.right);
}
}
- 迭代
使用栈
Java代码:
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> res = new LinkedList<>();
Stack<TreeNode> stack = new Stack<>();
while(null != root){
stack.push(root);
root = root.left;
}
while(!stack.isEmpty()){
TreeNode cur = stack.pop();
res.add(cur.val);
if(null != cur.right){
stack.push(cur.right);
while(null != stack.peek().left)
stack.push(stack.peek().left);
}
}
return res;
}
}