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Boundary of Binary Tree

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Description

https://leetcode.com/problems/boundary-of-binary-tree/

Solution

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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> boundaryOfBinaryTree(TreeNode* root) {
        vector<int> ret;
        if (!root) return ret;
        ret.push_back(root->val);
        if (!root->left && !root->right) return ret;
        
        TreeNode* traverseLeft = root->left;
        while(traverseLeft && (traverseLeft->left || traverseLeft->right) ) {
            ret.push_back(traverseLeft->val);
            traverseLeft = traverseLeft->left 
                ? traverseLeft->left : traverseLeft->right;
        }
        
        //Traverse all the left node
        traverseLeaf(ret, root);
        
        stack<int> st;
        TreeNode* traverseRight = root->right;
        //stack to store the right handside non-leaf node
        while (traverseRight && (traverseRight->right || traverseRight->left) ) {
            st.push(traverseRight->val);
            traverseRight = traverseRight->right 
                ? traverseRight->right : traverseRight->left;
        }
        
        while(!st.empty()) {
            ret.push_back(st.top());
            st.pop();
        }
        
        return ret;
    }
    
    void traverseLeaf(vector<int>& ret, TreeNode* root) {
        if (root == NULL) return;
        if (root->left == NULL && root->right == NULL) ret.push_back(root->val);
        traverseLeaf(ret, root->left);
        traverseLeaf(ret, root->right);
        
        return;
    }
};