binary-tree-postorder-traversal 递归与非递归后序遍历二叉树

递归

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> ret;
    vector<int> postorderTraversal(TreeNode *root) {
        if(!root) return ret;
        postorderTraversal(root->left);
        postorderTraversal(root->right);
        ret.push_back(root->val);
        return ret;
    }
};