二叉树中序遍历 2021.4.14

题目描述： 二叉树中序遍历

核心就是递归思想

代码如下：

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;
    }
}

public List<Integer> inorderTraversal(TreeNode root) {
    List<Integer> res = new ArrayList<Integer>();
    traverse(root, res);
    return res;
}

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