LeetCode—144. Binary Tree Preorder Traversal
题目
https://leetcode.com/problems/binary-tree-preorder-traversal/description/
前序遍历二叉树。
思路及解法
1.递归
2.非递归,栈
代码
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
List<Integer> list = new ArrayList<>();
public List<Integer> preorderTraversal(TreeNode root) {
preOrder(root);
return list;
}
public void preOrder(TreeNode node){
if(node == null){
return;
}
list.add(node.val);
if(node.left != null){
preOrder(node.left);
}
if(node.right != null){
preOrder(node.right);
}
}
}
/**
* 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> preorderTraversal(TreeNode root) {
List<Integer> list = new ArrayList<>();
if(root==null) return list;
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
while(!stack.isEmpty()){
TreeNode tmp = stack.pop();
list.add(tmp.val);
if(tmp.right!=null){
stack.push(tmp.right);
}
if(tmp.left!=null){
stack.push(tmp.left);
}
}
return list;
}
}