非递归
需要利用栈
一开始将root入栈;
循环结束条件:栈空;
每次循环,先使栈顶元素出栈,将其数值加入list中,再将其右结点入栈,左结点入栈,因为前序遍历是先根再左后右,入栈顺序要先右后左;
import java.util.ArrayList;
import java.util.Stack;
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public ArrayList<Integer> preorderTraversal(TreeNode root) {
ArrayList<Integer> a = new ArrayList<Integer>();
Stack<TreeNode> stack = new Stack<TreeNode>();
if (root == null){
return a;
}
stack.push(root);
while(!stack.isEmpty()){
TreeNode node = stack.pop();
a.add(node.val);
if(node.right != null){
stack.push(node.right);
}
if(node.left != null){
stack.push(node.left);
}
}
return a;
}
}
递归
import java.util.ArrayList;
import java.util.Stack;
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public ArrayList<Integer> preorderTraversal(TreeNode root) {
ArrayList<Integer> a = new ArrayList<Integer>();
if (root == null){
return a;
}
a.add(root.val);
if(root.left != null){
a.addAll(preorderTraversal(root.left));
}
if(root.right != null){
a.addAll(preorderTraversal(root.right));
}
return a;
}
}