5. Binary Tree Preorder Traversal(Recursive)
Oct 20, 2020
Given the root of a binary tree, return the preorder traversal of its nodes' values.
Example 1:
Input: root = [1,null,2,3]
Output: [1,2,3]Example 2:
Input: root = []
Output: []Example 3:
Input: root = [1]
Output: [1]Example 4:
Input: root = [1,2]
Output: [1,2]Example 5:
Input: root = [1,null,2]
Output: [1,2]Constraints:
- The number of nodes in the tree is in the range
[0, 100]. -100 <= Node.val <= 100
Solution
Given the root of a binary tree, return the preorder traversal of its nodes' values.
Example 1:
Input: root = [1,null,2,3]
Output: [1,2,3]Example 2:
Input: root = []
Output: []Example 3:
Input: root = [1]
Output: [1]Example 4:
Input: root = [1,2]
Output: [1,2]Example 5:
Input: root = [1,null,2]
Output: [1,2]Constraints:
- The number of nodes in the tree is in the range
[0, 100]. -100 <= Node.val <= 100
Solution
class Solution {
public List<Integer> preOrder(TreeNode root, List<Integer> sol) {
if(root == null)
return sol;
sol.add(root.val);
if(root.left != null)
sol = preOrder(root.left, sol);
if(root.right != null)
sol = preOrder(root.right, sol);
return sol;
}
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> sol = new ArrayList<>();
sol = preOrder(root, sol);
return sol;
}
}