DP-LeetCode-509. 斐波那契数

f[0] = 0 
f[1] = 1 
n>=2 f[n] = f[n - 1] + f[n - 2]

class Solution {
public:
    int fib(int n) {
        if (n == 0) {
            return 0;
        }
        if (n == 1) {
            return 1;
        }
        int a = 0;
        int b = 1;
        int c = 0;
        n -= 1;
        while (n--) {
            c = a + b;
            a = b;
            b = c;
        }
        return c;
    }
};