#leetcode509

The 2021 year will pass,it's so fast!

I should prepare the final exam,but i have to finish my homework before this friday.

I don't care my homework,and i think the knowledge that i should acquire is important.

Today,i write a leetcode509,because this is a dynamic programming type,and this is a simple problem about fibonacci-number.

What is fibonacci-number?
Each number is the sum of the two preceding ones,starting from 0 and 1.

/*

fib(0) = 0,fib(1) = 1

input:n = 2
fib(2) = fib(1) + fib(0)
output: 1

input:n = 3
fib(3) = fib(2) + fib(1)
output: 2

input:n = 4
fib(4) = fib(3) + fib(2)
output: 3

*/

My first submitting is pass by using recursion,but executive time is bad.

When i read haoel solution,and i want to say that it's amazing.
I write this problem again,and executive time beat 100% clients!

class Solution {
public:
    int fib(int n) {
        if (n < 2)
            return n;
        int first = 0;
        int second = 1;
        for (n -= 1; n > 0; n--)
        {
            second += first;
            first = second - first;//update the first value
        }
        return second;//return second value
    }
};

When i read carl solution,and i think that his thinking is also same haoel one.
His executive time also beat %100 clients,but his memory consuming is better than haoel one.

class Solution {
public:
    int fib(int n) {
        if (n <= 1)
            return n;
        int dp[2];
        dp[0] = 0;
        dp[1] = 1;
        for (int i = 2; i <= n; ++i)
        {
            int sum = dp[0] + dp[1];
            dp[0] = dp[1];
            dp[1] = sum;
        }
        return dp[1];
    }
};