House Robber III LeetCode solution in C++

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class Solution {
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 public:
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  vector<int> dp(TreeNode *root) {
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    vector<int> ans(2, 0); // dp[0]: maximal money you can get by robbing current node.
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                           // dp[1]: maximal money you can get when not rob this node
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    if (root == NULL) return ans;
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    vector<int> left = dp(root->left);
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    vector<int> right = dp(root->right);
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    ans[0] = root->val + left[1] + right[1];
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    ans[1] = max(left[0], left[1]) + max(right[0], right[1]);
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    return ans;
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  }
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  int rob(TreeNode *root) {
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    vector<int> ans = dp(root);
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    return max(ans[0], ans[1]);
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  }
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};