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-rw-r--r--other/nth_tribonacci_number.cpp61
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diff --git a/other/nth_tribonacci_number.cpp b/other/nth_tribonacci_number.cpp
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--- a/other/nth_tribonacci_number.cpp
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-// 1137. N-th Tribonacci Number
-#include "leetcode.h"
-
-/*
- * the tribonacci sequence t^n is defined as follows
- * t0 = 0, t1 = 1, t2 = 1, & tn+3 = tn + tn+1 + tn+2 for n >= 0
- * given n, return value of tn
- * eg.
- * n = 4, output: 4
- * t3 = 0 + 1 + 1 = 2
- * t4 = 1 + 1 + 2 = 4
- */
-
-typedef long long ll;
-struct Matrix {
- vvd(ll) a;
- int rows, cols;
- Matrix(int rows = 0, int cols = 0) : rows(rows), cols(cols) {
- a.resize(rows);
- for (int i = 0; i < rows; ++i)
- a[i].assign(cols, 0);
- }
- Matrix(const vvd(ll) & matrix)
- : a(matrix), rows(matrix.size()), cols(matrix[0].size()) {}
-};
-
-Matrix operator*(const Matrix &a, const Matrix &b) {
- Matrix res(a.rows, b.cols);
- for (int i = 0; i < a.rows; ++i)
- for (int j = 0; j < b.cols; ++j)
- for (int k = 0; k < a.cols; ++k)
- res.a[i][j] += a.a[i][k] * b.a[k][j];
- return res;
-}
-
-Matrix fastPow(const Matrix &matrix, int pow) {
- if (pow == 1)
- return matrix;
- Matrix half = fastPow(matrix, pow / 2);
- if (pow & 1)
- return half * half * matrix;
- return half * half;
-}
-
-class Solution {
-public:
- int tribonacci(int n) {
- if (n == 0)
- return 0;
- Matrix m({{0, 1, 0}, {0, 0, 1}, {1, 1, 1}});
- Matrix m0({{0}, {1}, {1}});
- Matrix mn = fastPow(m, n) * m0;
- return mn.a[0][0];
- }
-};
-
-int main() {
- Solution obj;
- cout << obj.tribonacci(4) << endl; // expect: 4
- cout << obj.tribonacci(25) << endl; // expect: 1389537
-}