二叉树前序遍历

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/**
 * Definition for a binary tree node.
 * 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) {
        vector<int> result;
        stack<TreeNode*> s;
        TreeNode* p = root;
        while(p || !s.empty())
        {
            if(p)
            {
                result.push_back(p->val);
                s.push(p);
                p = p->left;
            }
            else
            {
                p = s.top();
                s.pop();
                p = p->right;
            }
        }
        return result;
    }
};