1.PreOrder - Using Stack.txt

/**
 * 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:
    void preorder(TreeNode* root,vector<int> &ans)
    {
        if(root==NULL)
        {
            return;
        }
        else
        {
             ans.push_back(root->val);
            preorder(root->left,ans);
            preorder(root->right,ans);
        }
    }
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> ans;
        preorder(root,ans);
        return ans;
    }
};




Using stack

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