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·98 lines (77 loc) · 1.87 KB
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E
方法1: DP array.
方法1.1: 滚动数组, 简化DP。
方法2: recursively calculate fib(n - 1) + fib(n - 2). 公式没问题, 但是时间太长, timeout.
```
/*
Find the Nth number in Fibonacci sequence.
A Fibonacci sequence is defined as follow:
The first two numbers are 0 and 1.
The i th number is the sum of i-1 th number and i-2 th number.
The first ten numbers in Fibonacci sequence is:
0, 1, 1, 2, 3, 5, 8, 13, 21, 34 ...
Example
Given 1, return 0
Given 2, return 1
Given 10, return 34
Note
The Nth fibonacci number won't exceed the max value of signed 32-bit integer in the test cases.
Tags Expand
Enumeration Mathematics Non Recursion
*/
/*
Recap 3.28.2016.
Rolling array, instead of initiating array.
*/
class Solution {
public int fibonacci(int n) {
if (n <= 1) {
return 0;
}
int first = 0;
int second = 1;
for (int i = 2; i < n; i++) {
int temp = second;
second = first + second;
first = temp;
}
return second;
}
}
/*
Thoughts:
Bottomup with for loop.
1. If non-recursion, do for loop for that n
2. Note: this specfiic problem is not 0-based. it's 1-based.
3. return fib[n]
*/
class Solution {
public int fibonacci(int n) {
if (n <= 1) {
return 0;
}
int[] fib = new int[n];
fib[0] = 0;
fib[1] = 1;
for (int i = 2; i < n; i++) {
fib[i] = fib[i - 1] + fib[i - 2];
}
return fib[n - 1];
}
}
/*
Recursive. Long time complexity
Timeout
*/
class Solution {
public int fibonacci(int n) {
if (n <= 1) {
return 0;
}
if (n == 2) {
return 1;
}
return fibonacci(n - 1) + fibonacci(n - 2);
}
}
```