leetcode 509. Fibonacci number
Question stem
Fibonacci number, usually expressed by F(n), the sequence formed is called Fibonacci sequence. The number sequence starts with 0 and 1, and each number after it is the sum of the previous two numbers. That is:
F(0) = 0, F(1) = 1
F(n) = F(n-1) + F(n-2), where n> 1
gives you n, please calculate F(n).
Example 1:
Input: 2
Output: 1
Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1
Example 2:
Input: 3
Output: 2
Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2
Example 3:
Input: 4
Output: 3
Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3
Prompt:
0 <= n <= 30
answer
You can directly hit the table
class Solution {
public:
int fib(int n) {
vector<int> fib(31,0);
fib[0] = 0;
fib[1] = 1;
for(int i = 2 ; i < 31 ; ++i){
fib[i] = fib[i - 1] + fib[i - 2];
}
return fib[n];
}
};
You can also use two variables to store adjacent two bits to save space
class Solution {
public:
int fib(int n) {
if(n < 2){
return n;
}
int first = 0;
int second = 1;
for(int i = 1 ; i < n ; ++i){
int temp = first;
first = second;
second = temp + second;
}
return second;
}
};