LintCode-66.二叉树的前序遍历

二叉树的前序遍历

给出一棵二叉树，返回其节点值的前序遍历。

样例

给出一棵二叉树 {1,#,2,3},

返回 [1,2,3].

挑战

你能使用非递归实现么？

标签

递归 二叉树 二叉树遍历 非递归

code

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: Preorder in vector which contains node values.
     */
    vector<int> preorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> order;
        if(root == NULL)
            return order;

        stack<TreeNode*> s;
        TreeNode *p=root;
        while(p!=NULL||!s.empty()) {
            while(p!=NULL) {
                order.push_back(p->val);
                s.push(p);
                p=p->left;
            }
            if(!s.empty()) {
                p=s.top();
                s.pop();
                p=p->right;
            }
        }
        return order;
    }
};