LeetCode - Binary Tree Preorder Traversal

Binary Tree Preorder Traversal

2014.1.14 02:36

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

Solution1:

　　The recursive version of preorder traversal, everybody knows what to do.

　　Time and space complexities are both O(n), where n is the number of nodes in the tree.

Accepted code:

// 1WA, 1RE, 1AC
/**
 * 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.
        result.clear();
        // 1WA here, this sentence missing
        preorder(root);
        return result;
    }
private:
    vector<int> result;
    
    void preorder(TreeNode *root) {
        // 1RE here, if(root)
        if(root == nullptr){
            return;
        }
        result.push_back(root->val);
        preorder(root->left);
        preorder(root->right);
    }
};

Solution2:

　　Iterative version, not so easy as the last one.

　　Time and space complexities are still O(n).

Accepted code:

class Solution {
public:
    vector<int> preorderTraversal(TreeNode *root) {
        result.clear();
        
        if(root == nullptr){
            return result;
        }
        
        vstack.clear();
        cstack.clear();
        vstack.push_back(root);
        cstack.push_back(0);
        result.push_back(root->val);
        while(vstack.size() > 0){
            if(cstack[cstack.size() - 1] == 0){
                ++cstack[cstack.size() - 1];
                if(vstack[vstack.size() - 1]->left != nullptr){
                    cstack.push_back(0);
                    vstack.push_back(vstack[vstack.size() - 1]->left);
                    result.push_back(vstack[vstack.size() - 1]->val);
                }
            }else if(cstack[cstack.size() - 1] == 1){
                ++cstack[cstack.size() - 1];
                if(vstack[vstack.size() - 1]->right != nullptr){
                    cstack[cstack.size() - 1] = 0;
                    vstack[vstack.size() - 1] = vstack[vstack.size() - 1]->right;
                    result.push_back(vstack[vstack.size() - 1]->val);
                }
            }else{
                cstack.pop_back();
                vstack.pop_back();
            }
        }
        
        return result;
    }
private:
    vector<int> result;
    vector<TreeNode *> vstack;
    vector<int> cstack;
};