19.2.24 [LeetCode 94] Binary Tree Inorder Traversal

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]

class Solution {
public:
    void build(TreeNode*root, vector<int>&ans) {
        if (root == NULL)return;
        build(root->left, ans);
        ans.push_back(root->val);
        build(root->right, ans);
    }
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int>ans;
        build(root, ans);
        return ans;
    }
};