Number of Ways to Stay in the Same Place After Some Steps LeetCode solution in C++

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class Solution {
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 public:
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  int n, MOD = 1e9 + 7;
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  int numWays(int steps, int arrLen) {
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    n = arrLen;
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    vector<vector<int>> memo(steps / 2 + 1, vector<int>(steps + 1, -1));
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    return dp(memo, 0, steps) % MOD;
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  }
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  long dp(vector<vector<int>> &memo, int i, int steps) {
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    if (steps == 0) return i == 0;
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    if (i < 0 || i == n || i > steps) return 0;
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    if (memo[i][steps] != -1) return memo[i][steps];
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    return memo[i][steps] =
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               (dp(memo, i, steps - 1) + dp(memo, i - 1, steps - 1) + dp(memo, i + 1, steps - 1)) %
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               MOD;
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  }
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};