Problem:
Score : 300 points

Problem Statement

There is a staircase with N steps. Takahashi is now standing at the foot of the stairs, that is, on the 0-th step.
He can climb up one or two steps at a time.

However, the treads of the a_1-th, a_2-th, a_3-th, \ldots, a_M-th steps are broken, so it is dangerous to set foot on those steps.

How many are there to climb up to the top step, that is, the N-th step, without setting foot on the broken steps?
Find the count modulo 1\ 000\ 000\ 007.

Constraints

1 \leq N \leq 10^5

0 \leq M \leq N-1

1 \leq a_1 < a_2 < ... < a_M \leq N-1

Input

Input is given from Standard Input in the following format:

N M
a_1
a_2
.
.
.
a_M

Output

Print the number of ways to climb up the stairs under the condition, modulo 1\ 000\ 000\ 007.

Sample Input 1

6 1
3

Sample Output 1

4

There are four ways to climb up the stairs, as follows:

0 \to 1 \to 2 \to 4 \to 5 \to 6

0 \to 1 \to 2 \to 4 \to 6

0 \to 2 \to 4 \to 5 \to 6

0 \to 2 \to 4 \to 6

Sample Input 2

10 2
4
5

Sample Output 2

0

There may be no way to climb up the stairs without setting foot on the broken steps.

Sample Input 3

100 5
1
23
45
67
89

Sample Output 3

608200469

Be sure to print the count modulo 1\ 000\ 000\ 007.

Solution in Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

const (
	initialBufSize = 10000
	maxBufSize     = 100000000
)

var sc *bufio.Scanner = func() *bufio.Scanner {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	// buf := make([]byte, initialBufSize)
	// sc.Buffer(buf, maxBufSize)

	return sc
}()

func nextInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func nextInts(n int) []int {
	ints := make([]int, n)
	for i := 0; i < n; i++ {
		ints[i] = nextInt()
	}
	return ints
}

const divisor = 1000000007

type brokenIndexes map[int]struct{}

func (bi *brokenIndexes) add(key int) {
	(*bi)[key] = struct{}{}
}

func countWays(stairNum int, bi brokenIndexes) int {
	counts := make([]int, stairNum+1)
	counts[0] = 1
	counts[1] = returnStairs(1, 1, bi)
	for i := 2; i < stairNum+1; i++ {
		stairs := counts[i-1] + counts[i-2]
		if stairs == 0 {
			return 0
		}
		counts[i] = returnStairs(i, stairs, bi) % divisor
	}
	return counts[stairNum]
}

func returnStairs(index, value int, bi brokenIndexes) int {
	if _, ok := bi[index]; ok {
		return 0
	}
	return value
}

func main() {
	n, m := nextInt(), nextInt()
	bi := make(brokenIndexes, m)
	for i := 0; i < m; i++ {
		bi.add(nextInt())
	}
	r := countWays(n, bi)
	fmt.Println(r)
}