/*
* public class TreeNode {
* int val = 0;
* TreeNode left = null;
* TreeNode right = null;
* public TreeNode(int val) {
* this.val = val;
* }
* }
*/
public class Solution {
/**
* 代码中的类名、方法名、参数名已经指定,请勿修改,直接返回方法规定的值即可
*
*
* @param root TreeNode类
* @return int整型一维数组
*/
public int[] preorderTraversal (TreeNode root) {
// write code here
//第一步 用list保存前序遍历结果
//第二步 转为int[] 返回
ArrayList<Integer> list = new ArrayList<>();
pre(root,list);
int[] preArray = new int[list.size()];
for(int i = 0;i<list.size();i++){
preArray[i] = list.get(i);
}
return preArray;
}
private void pre(TreeNode root,ArrayList<Integer> list){
if(root == null){
return;
}
//前中后只是调整下面三行的顺序即可
list.add(root.val);
pre(root.left,list);
pre(root.right,list);
}
}
// =============== 非递归方式 =====================
public static void preOrder(Node head,ArrayList<Integer> list){
if(head == null){
return;
}
System.out.println("先序遍历,非递归方式");
Stack<Node> s = new Stack<Node>();
s.push(head);
while( !s.isEmpty() ){
head = s.pop();
list.add(head.val);
if(head.right != null){
s.push(head.right);
}
if(head.left != null){
s.push(head.left);
}
}
}