树-前序遍历之递归写法

一、前序遍历的函数实现

import java.util.ArrayList;
import java.util.List;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    // 构造函数
    TreeNode() {}
    TreeNode(int val)
    { this.val = val; }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

public class Solution2 {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        preorder(root, res);
        return res;
    }

    public static void preorder(TreeNode root, List<Integer> res){
        if(root == null)return;
        res.add(root.val);
        preorder(root.left,res);
        preorder(root.right,res);
    }

}

二、构造树等测试部分

import java.util.List;

public class test {
    public static void main(String[] args) {
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(3);
        TreeNode node4 = new TreeNode(4);
        TreeNode node5 = new TreeNode(5);
        TreeNode node6 = new TreeNode(6);
        TreeNode node7 = new TreeNode(7);
        node1.left = node2;node1.right = node3;
        node2.left = node4;node2.right = node5;
        node3.left = node6;node3.right = node7;
        // 这里要用 List 接收，如果使用 ArrayList 接收则需要强制转型
        List<Integer> res = new Solution2().preorderTraversal(node1);
        for (int i = 0; i < res.size(); i++) {
            // 这里需要用到 get 函数
            System.out.println(res.get(i));
        }
    }
}