// 1269. Number of Ways to Stay in the Same Place After Some Steps #include "leetcode.h" /* * you have a pointer at index 0 in an array of size 'arr_len'. at each step, * you can move 1 position to the left, 1 position to the right in the array, or * stay in the same plae. given two integers 'steps' and 'arr_len' return the * number of ways such that your pointer is still at index 0 after exactly * 'steps' steps. since the answer may be too large, return it modulo 10e9+7 */ class Solution { public: int numWays(int steps, int arr_len) { int n = min(steps / 2 + 1, arr_len); vector v1(n + 2), v2(n + 2); v1[1] = 1; while (steps--) { for (auto i = 1; i <= n; ++i) v2[i] = ((long)v1[i] + v1[i - 1] + v1[i + 1]) % 1000000007; swap(v1, v2); } return v1[1]; } }; int main() { Solution obj; printf("%d\n", obj.numWays(3, 2)); // expect: 4 printf("%d\n", obj.numWays(2, 4)); // expect: 2 printf("%d\n", obj.numWays(4, 2)); // expect: 8 }