[数据结构与算法]Leetcode 二叉树的后序遍历

迭代：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
typedef pair<int, TreeNode*> P;
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
         vector<int> out;
         stack<P> stk;
         stk.push(P(0, root));
         while(stk.size() > 0) {
             P temp = stk.top();
             stk.pop();
             if(!temp.second) continue;
             if(!temp.first) {
                 stk.push(P(1, temp.second));
                 stk.push(P(0, temp.second->right));
                 stk.push(P(0, temp.second->left));
             } else {
                 out.push_back(temp.second->val);
             }
         }
         return out;
    }
};

递归：

class Solution {
    void postorder(vector<int>& ans, TreeNode* tree){
        if(!tree) return;
        postorder(ans, tree->left);
        postorder(ans, tree->right);
        ans.push_back(tree->val);
    }
public:
    vector<int> postorderTraversal(TreeNode* root) {
         vector<int> out;
         postorder(out, root);
         return out;
    }
};