【atcoder beginner 308E - MEX】

• 前缀和
• 二分查找
• 打表枚举
• 代码如下

• import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.StreamTokenizer;
  import java.util.ArrayList;
  import java.util.List;
  
  public class Main {
  
      public static Integer[] dp(int[] arr, String s,int t,char ch) {
          List<Integer> ans = new ArrayList<>();
          if (arr[0] == t && s.charAt(0) == ch) {
              ans.add(1);
          } else {
              ans.add(0);
          }
          for (int i = 1; i < arr.length; i++) {
              if (arr[i] == t && s.charAt(i) == ch) {
                  ans.add(ans.get(i - 1) + 1);
              } else {
                  ans.add(ans.get(i - 1));
              }
          }
          return ans.toArray(new Integer[ans.size()]);
      }
  
      public static void main(String[] args) throws IOException {
          StreamTokenizer in = new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in)));
          in.nextToken();
          int n = (int) in.nval;
          int[] arr = new int[n];
          for (int i = 0; i < arr.length; i++) {
              in.nextToken();
              arr[i] = (int) in.nval;
          }
          in.nextToken();
          String s = in.sval;
          Integer[] mlist = build(s, 'M');
          Integer[] elist = build(s, 'E');
          Integer[] xlist = build(s, 'X');
  
          Integer[] hm0 = dp(arr, s,0,'M');
          Integer[] hm1 = dp(arr, s,1,'M');
          Integer[] hm2 = dp(arr,s, 2,'M');
  
          Integer[] hx0 = dp(arr, s,0,'X');
          Integer[] hx1 = dp(arr, s,1,'X');
          Integer[] hx2 = dp(arr,s, 2,'X');
  
          long ans = 0;
          for (int i = 0; i < elist.length; i++) {
              int j = upperlound(mlist, elist[i]);
              int p = upperlound(xlist, elist[i]);
  
              if (j == 0) {
                  continue;
              }
              if (p == xlist.length) {
                  continue;
              }
              j = j - 1;
  
              int t = mlist[j];
  
              int x0 = hm0[t];
              int x1 = hm1[t];
              int x2 = hm2[t];
  
              int k = xlist[p];
  
              int y0 = k > 0 ? hx0[n - 1] - hx0[k - 1] : hx0[n - 1];
              int y1 = k > 0 ? hx1[n - 1] - hx1[k - 1] : hx1[n - 1];
              int y2 = k > 0 ? hx2[n - 1] - hx2[k - 1] : hx2[n - 1];
  
              int z = arr[elist[i]];
  
              if (z == 2) {
                  ans += 1l*x0*(y0 + y2 + 3 * y1);
                  ans += 3l*x1*y0;
                  ans += 1l* x2*y0;
              }
              else if( z == 1){
                  ans += 1l*x0*(2*y0+2*y1+3*y2);
                  ans += 2l*x1*y0;
                  ans += 3l*x2*y0;
              }
              else{
                  ans += 1l*x0*(y0+2*y1+y2);
                  ans += 1l*x1*(2*y0+2*y1+3*y2);
                  ans += 1l*x2*(y0+3*y1+y2);
              }
  
          }
  
          System.out.println(ans);
      }
  
      /**
       * @param arr
       * @param value
       * @return 》 value的第一个坐标位置，如果不存在，那么就是arr.length;
       */
      public static int upperlound(Integer[] arr, int value) {
          int low = 0;
          int high = arr.length;
          while (low < high) {
              int mid = low + (high - low) / 2;
              if (arr[mid] <= value) {
                  low = mid + 1;
              } else {
                  high = mid;
              }
          }
          return low;
      }
  
      public static Integer[] build(String str, char ch) {
          List<Integer> list = new ArrayList<>();
          for (int i = 0; i < str.length(); i++) {
              if (str.charAt(i) == ch) {
                  list.add(i);
              }
          }
          return list.toArray(new Integer[list.size()]);
      }
  
  }