leetcode 509. Fibonacci Number

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比较经典的算法题，Fibonacci序列：

solution 1 :递归

class Solution {
public:
    int fib(int N) {
        if(N<=1){
            return N;
        }
        return fib(N-1) + fib(N-2);
        
    }
};

solution2:DP

class Solution {
public:
    int fib(int N) {
        if(N<=1){
            return N;
        }
        
        int ret[N+1];
        ret[0] = 0;
        ret[1] = 1;
        for(int i = 2;i<=N;i++){
            ret[i] = ret[i-1] + ret[i-2];
        }
        return ret[N];
    }
};

solution3:iterative method

class Solution {
public:
    int fib(int N) {
        if(N<2){
            return N;
        }
        int a = 0;
        int b = 1;
        int c = 0;
        for(int i = 2;i<=N;i++){
            c = a + b;
            a = b;
            b = c;
        }
        return c;
        
    }
};