LeetCode: Binary Tree Preorder Traversal http://oj.leetcode.com/problems/binary-tree-preorder-traversal/

binary_tree_preorder_traversal.cpp

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode *root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        vector<int> res;
        if (root == NULL)
            return res;
            
        stack<TreeNode *> st;
        st.push(root);
        
        while (!st.empty()) {
            TreeNode *cur = st.top();
            st.pop();
            res.push_back(cur->val);
            if (cur->right)
                st.push(cur->right);
            if (cur->left)
                st.push(cur->left);
        }
        return res;
    }
};