使用循环

class Solution {
    public int fib(int n) {
        if(n == 0){
            return 0;
        }
        if(n == 1){
            return 1;
        }
        int res = 0;
        int pre1 = 1;
        int pre2 = 0;
        for(int i = 2; i <= n; i++){
            res = pre1 + pre2;
            pre2 = pre1;
            pre1 = res;
        }
        return res;
    }
}

使用HashMap

class Solution {
    private Map<Integer,Integer> storeMap = new HashMap();
    public int fib(int n) {
        if(n == 0){
            return 0;
        }
        if(n == 1){
            return 1;
        }
        if(null != storeMap.get(n)){
            return storeMap.get(n);
        }else{
            int res = fib(n-1) + fib(n-2);
            storeMap.put(n,res);
            return res;
        }
    }
}