145.binary-tree-postorder-traver

迭代方法
栈、队列

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        if(root == nullptr)
            return {};
        
        deque<int> ans;
        stack<TreeNode*> s;
        s.push(root);
        while(!s.empty()){
            TreeNode* curNode = s.top();s.pop();
            ans.push_front(curNode->val);
            if(curNode->left) s.push(curNode->left);
            if(curNode->right) s.push(curNode->right);
        }
        return vector<int>(ans.begin(), ans.end());
    }
};