144. Binary Tree Preorder Traversal
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given a binary tree, return the preorder traversal of its nodes‘ values.
Solution 1://非递归
public class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> result = new ArrayList<>();
if (root == null) {
return result;
}
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
while (!stack.empty()) {
TreeNode node = stack.pop();
result.add(node.val);
if (node.right != null) {
stack.push(node.right);
}
if (node.left != null) {
stack.push(node.left);
}
}
return result;
}
}
Solution 2：//递归
public class Solution {
List<Integer> result = new ArrayList<>();
public List<Integer> preorderTraversal(TreeNode root) {
if (root != null)
helper(root);
return result;
}
private void helper(TreeNode root) {
result.add(root.val);
if (root.left != null)
helper(root.left);
if (root.right != null)
helper(root.right);
}
}
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