給出一棵二叉樹,返回其中序遍歷

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/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Inorder in vector which contains node values.
     */
public:
    void inorder_tra(TreeNode * p,vector<int> & vi){
        if(p!=NULL){
            inorder_tra(p->left,vi);
            vi.push_back(p->val);
        
            inorder_tra(p->right,vi);
        }
        
    }
    vector<int> inorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> vi;
        inorder_tra(root,vi);
        return vi;
    }
};