LeeCode:144. Binary Tree Preorder Traversal

题目描述

Given a binary tree, return the preorder traversal of its nodes’ values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

Follow up: Recursive solution is trivial, could you do it iteratively?

解题思路

本题要求不能用递归实现。可以用栈来模拟递归。

AC 代码

/** * 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 {
    // 递归实现前序遍历
     void preorderTraversalRecursive(TreeNode* root, vector<int>& preSeq) 
     {
         if(root == nullptr) return;

         preSeq.push_back(root->val);
         // 遍历左右孩子
         preorderTraversalRecursive(root->left, preSeq);
         preorderTraversalRecursive(root->right, preSeq);
     }
    // 非递归实现前序遍历
    void preorderTraversalWithoutRecursive(TreeNode* root, vector<int>& preSeq) {
        stack<TreeNode*> stk;
        stk.push(root);

        while(!stk.empty())
        {
            TreeNode* curNode = stk.top();
            stk.pop();

            if(curNode == nullptr) continue;
            preSeq.push_back(curNode->val);

            // 遍历左右孩子
            stk.push(curNode->right);
            stk.push(curNode->left);
        }
    }
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> preSeq;

        //preorderTraversalRecursive(root, preSeq);
        preorderTraversalWithoutRecursive(root, preSeq);

        return preSeq;
    }
};