树与二叉树

二叉树的结构

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

二叉树的递归遍历算法

先序遍历【递归】

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

    }

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