leetcode:Binary Tree Preorder Traversal 先序遍历

最新推荐文章于 2022-03-03 21:24:03 发布

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本文介绍了一种不使用递归的方法来完成二叉树节点值的前序遍历。通过栈数据结构实现了迭代式的解决方案，并提供了一个具体的实现示例。此方法避免了递归带来的潜在堆栈溢出风险。

/**
 * Given a binary tree, return the preorder traversal of its nodes' values.
 * Note: Recursive solution is trivial, could you do it iteratively?

*/

/// Test Unit
/* 
 *          1
 *         / \
 *        2   3
 *           /
 *          4
 *           \
 *            5
 */

#include <iostream>
#include <stack>
#include <vector>
using namespace std;

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution
{
public:
    Solution(){};
    ~Solution(){};

    vector<int> preorderTraversal(TreeNode* root){
        vector<int> res;
        stack<TreeNode*> s;

        if(!root)
            return res;

        s.push(root);
        while(!s.empty()){
            TreeNode* p=s.top();
            res.push_back(p->val);
            s.pop();
            if(p->right) s.push(p->right);
            if(p->left) s.push(p->left);
        }

        return res;
    }

};


int main(int argc, char** argv) {
    TreeNode root(1);
    TreeNode node2(2);
    TreeNode node3(3);
    TreeNode node4(4);
    TreeNode node5(5);

    root.left = &node2;
    root.right = &node3;
    node3.left = &node4;
    node4.right = &node5;

    Solution s;
    for (auto i : s.preorderTraversal(&root))
        cout << i << " ";
    cout << endl; 

    return 0;
}