Binary Tree Right Side View LeetCode solution in C++

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/**
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 * Definition for a binary tree node.
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 * struct TreeNode {
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 *     int val;
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 *     TreeNode *left;
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 *     TreeNode *right;
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 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
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 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
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 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
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 * right(right) {}
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 * };
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 */
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class Solution {
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 public:
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  void solve(TreeNode *root, vector<int> &ans, int level) {
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    // base case
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    if (root == NULL) {
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      return;
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    }
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    // we entered a new level
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    if (level == ans.size()) {
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      ans.push_back(root->val);
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    }
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    solve(root->right, ans, level + 1);
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    solve(root->left, ans, level + 1);
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  }
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  vector<int> rightSideView(TreeNode *root) {
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    vector<int> ans;
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    int level = 0;
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    solve(root, ans, level);
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    return ans;
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  }
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};