【树】leetcode_145_ 二叉树的后序遍历

【树】leetcode_145_ 二叉树的后序遍历

c++递归

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> re;
        get_re(root, re);
        return re;
    }

    void get_re(TreeNode *root, vector<int> &re){
        if(root==nullptr){
            return;
        }
        get_re(root->left, re);
        get_re(root->right, re);
        re.push_back(root->val);
    }
};

迭代（把先序的《中左右》变成《中右左》，然后翻转就是后序的《左右中》）

/**
 * 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) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> result;
        stack<TreeNode*> re;

        while(root != nullptr || !re.empty()){
            while(root != nullptr){
                result.push_back(root->val);
                re.push(root);
                root = root->right;
            }
            // if(!re.empty()){
                root = re.top();
                re.pop();
                root = root->left;

            // }
        }
        reverse(result.begin(),result.end());
        return result;

    }
};