144. Binary Tree Preorder Traversal

2019年1月26日 198次阅读 来源: ranjiewen

144. Binary Tree Preorder Traversal

题目

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

For example:
Given binary tree [1,null,2,3],

   1
    \
     2
    /
   3

return [1,2,3].

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

非递归实现二叉树的后序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

//preorder tranversal
class Solution {
public:
    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> vec;
        stack<TreeNode*> sta;
        if (root)
        {
            /*vec.push_back(root->val);
            sta.push(root);
            root = root->left;*/
            while (root||!sta.empty()) // && bug1  //树不为空或者栈不为空，继续循环
            {
                while (root)
                {
                    vec.push_back(root->val); //第一次遍历根节点
                    sta.push(root);
                    root = root->left;
                }
                if (!sta.empty())
                {
                    TreeNode* temp = sta.top();
                    sta.pop();
                    root = temp->right;
                }
            }
        }
        return vec;
    }
};

144. Binary Tree Preorder Traversal

    原文作者：ranjiewen
    原文地址: https://www.cnblogs.com/ranjiewen/p/8080933.html
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