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//递归解法：
class Solution {
public:
    void pTra(TreeNode* root, vector<int> &vec) {
        if (root == NULL) return;
        pTra(root->left, vec);
        pTra(root->right, vec);
        vec.insert(vec.end(), root->val);
    }
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> result;
        pTra(root, result);
        return result;
    }
};