# Common algorithm: Fibonacci sequence

2022-01-26 22:24:14 Yan Long

## subject ：

Write a function , Input n , Fibonacci, please （Fibonacci） The number of the sequence n term （ namely F(N)）. Fibonacci series is defined as follows ： F(0) = 0, F(1) = 1 F(N) = F(N - 1) + F(N - 2), among N > 1.

class Solution {
public:

map<int, int> cache;
int fib(int n) {
if (n == 0 || n == 1) {
return n;
}

if (cache.find(n) != cache.end()) {
return cache[n];
}

int res = (fib(n - 1) + fib(n - 2)) % 1000000007;
cache[n] = res;

return res;
}
};

Answer key 2： belt dp Dynamic programming of memorandum

class Solution {
public:

int fib(int n) {
if (n == 0 || n == 1) {
return n;
}

if(n == 2) {
return 1;
}

vector<int> dp(n + 1, 0);
// base case
dp[1] = dp[2] = 1;
for (int i = 3; i < n + 1; i++) {
dp[i] = dp[i - 2] + dp[i - 1];
}

return dp[n];
}
};

tips: 1、 When using recursion , You need to cache the results of the previous addition 2、 The addition result needs to be correct 1000000007 Remainder operation , Prevent the result from exploding .

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