Leetcode 144 (three methods)

I learned tree data structure again and watched the video by IIT professor.

Leetcode 144 solved by recursion is very simple to understand, but i try used

other way to conquer it.

/**
 * 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) {}
 * };
 */

Method one : RECURSION

class Solution {
public:
    void Traversal(TreeNode* cur, vector<int>& re) {
        if (cur == NULL) return;
        re.push_back(cur -> val);
        Traversal(cur -> left, re);
        Traversal(cur -> right, re);
    }
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> ret;
        Traversal(root,ret);
        return ret;
    }
};

Method two : BFS

class Solution {
public:
    void bfs(TreeNode* cur, queue<int>& q, vector<int>& r)     {
        if (!cur) return ;
        q.push(cur -> val);
        bfs(cur->left,q,r);
        bfs(cur->right,q,r);
        r.push_back(q.front());
        q.pop();
    }
    vector<int> preorderTraversal(TreeNode* root) {
        queue<int> q;
        vector<int> ret;
        bfs(root, q, ret);
        return ret;
    }
};

Method three : STACK

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        stack<TreeNode*> stk;
        vector<int> ret;
        if (root) stk.push(root);
        while (!stk.empty()) {
            TreeNode *node = stk.top();
            ret.push_back(node -> val);
            stk.pop();
            if (node -> right) stk.push(node -> right);
            if (node -> left) stk.push(node -> left);
        }
        return ret;
    }
};

KEEP GOING!