二叉树前序遍历
给出一棵二叉树,返回其节点值的前序遍历。
给出一棵二叉树 {1,#,2,3}
,
1 \ 2 / 3
返回 [1,2,3]
.
二叉树的前序遍历:根节点->左子树->右子树。下图的前序遍历结果为:abdefgc。
通过递归方式实现前序遍历的代码如下:
/**
* Definition of TreeNode:
* public class TreeNode {
* public int val;
* public TreeNode left, right;
* public TreeNode(int val) {
* this.val = val;
* this.left = this.right = null;
* }
* }
*/
public class Solution {
/**
* @param root: The root of binary tree.
* @return: Preorder in ArrayList which contains node values.
*/
ArrayList<Integer> list = new ArrayList<Integer>();
public ArrayList<Integer> preorderTraversal(TreeNode root) {
// write your code here
preorder(root);
return list;
}
private void preorder(TreeNode root){
if(root == null) return;
list.add(root.val);
preorder(root.left);
preorder(root.right);
}
}
通过非递归方式实现前序遍历的代码如下:
/**
* Definition of TreeNode:
* public class TreeNode {
* public int val;
* public TreeNode left, right;
* public TreeNode(int val) {
* this.val = val;
* this.left = this.right = null;
* }
* }
*/
public class Solution {
/**
* @param root: The root of binary tree.
* @return: Preorder in ArrayList which contains node values.
*/
public ArrayList<Integer> preorderTraversal(TreeNode root) {
// write your code here
Stack<TreeNode> stack = new Stack<TreeNode>();
ArrayList<Integer> list = new ArrayList<Integer>();
while(root!=null||stack.size()>0){
while(root!=null){
list.add(root.val);
stack.push(root);
root = root.left;
}
if(stack.size()>0){
root = stack.pop();
root = root.right;
}
}
return list;
}
}