题解 | #路由器#

import java.util.Scanner;

public class Main {
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);

        while(in.hasNext()){
            solution(in);
        }
    }

    /**
     * 差分数组
     *
     * 原数组[a1, a2, a3, a4, ...]
     * 差分数组[a1, a2-a1, a3-a2, a4-a3, ...]
     * 原数组第i项的值等于差分数组前i项的和
     *
     * 假如需要在区间[i,j]范围内都进行+1操作
     * 对于原数组, 需要遍历第i到j项都进行+1, 时间复杂度大
     * 对于差分数组, 只需在第i项+1, 第j+1项-1就行, 时间复杂度小
     *
     * @param in
     */
    private static void solution(Scanner in){
        int n = in.nextInt();
        int k = in.nextInt();

        int[] routers = new int[n];
        for(int i=0; i<n; i++){
            routers[i] = in.nextInt();
        }

        // 差分数组
        int[] diff = new int[n+1];
        int left,right;
        for(int i=0; i<n; i++){
            left = Math.max(0, i-routers[i]);
            right = Math.min(n, i+routers[i]+1);
            diff[left]++;
            diff[right]--;
        }

        int result = 0;
        int[] sum = new int[n];
        for(int i=0; i<n; i++){
            if(i > 0){
                sum[i] = sum[i-1] + diff[i];
            }else{
                sum[i] = diff[0];
            }
            if(sum[i] >= k){
                result++;
            }
        }

        System.out.println(result);
    }
}