Leetcode | 144. Binary Tree Preorder Traversal

144. Binary Tree Preorder Traversal(二叉数的前序遍历)

前序遍历：根节点->左子树->右子树

方法1: 非递归法

/**
 * 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> temp;
        if(root == nullptr)
            return temp;
        TreeNode *p;
        stack<TreeNode*> st;
        st.push(root);
        while(!st.empty())
        {
            p = st.top();
            st.pop();
            temp.push_back(p->val);
            if(p->right)
                st.push(p->right);
            if(p->left)
                st.push(p->left);
        }
        return temp;
    }
};