【LeetCode】94. Binary Tree Inorder Traversal

原题

Given a binary tree, return the inorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,3,2]

难度：中等

题意解析：其实就是二叉树的中序遍历

解法：递归

步骤

1) 递归左子树

2) 将当前节点的值插入列表

3) 递归右子树
 

    public List inorderTraversal(TreeNode root) {
        if(root == null) return new ArrayList<>();
        List result = new ArrayList<>();
        recursion(root.left, result);
        result.add(root.val);
        recursion(root.right, result);
        return result;
    }

    public void recursion(TreeNode node, List result)
    {
        if(node == null) return;
        if(node.left != null)
        {
            recursion(node.left, result);
        }

        result.add(node.val);
        if(node.right != null)
        {
            recursion(node.right, result);
        } 
    }