数组范围查询频率与值相同的元素

2021年3月20日16:54:12 发表评论 811 次浏览

本文概述

给定一个由N个数字组成的数组, 任务是回答以下类型的Q个查询：

query(start, end) = Number of times a 
number x occurs exactly x times in a 
subarray from start to end

例子：

输入：arr = {1, 2, 2, 2, 3, 3, 3}查询1：开始= 0, 结束= 1, 查询2：开始= 1, 结束= 1, 查询3：开始= 0, 结束= 2, 查询4：开始= 1, 结束= 3, 查询5：开始= 3, 结束= 5, 查询6：开始= 0, 结束= 5输出：1 0 2 1 1 3说明在查询1中, 元素1出现一次子数组[1, 2];在查询2中, 没有元素满足要求的条件是子数组[2]；在查询3中, 元素1在子数组[1、2、2]中发生一次, 而元素2发生两次。在查询4中, 元素2在子数组[2, 2, 3]中出现两次；在查询5中, 元素3在子数组[3、3、3]中出现三次；在查询6中, 元素1在子数组[1、2、2、3、3、3]中发生一次, 2发生两次, 3发生三次

推荐：请尝试以下方法{IDE}首先, 在继续解决方案之前。

方法1(蛮力)

计算每个查询下子数组中每个元素的频率。如果在每个查询覆盖的子数组中任何数字x的频率为x, 我们将增加计数器。

C ++

/* C++ Program to answer Q queries to 
    find number of times an element x 
    appears x times in a Query subarray */
#include <bits/stdc++.h>
using namespace std;
  
/* Returns the count of number x with
    frequency x in the subarray from 
    start to end */
int solveQuery( int start, int end, int arr[])
{
     // map for frequency of elements
     unordered_map< int , int > frequency;
  
     // store frequency of each element 
     // in arr[start; end]
     for ( int i = start; i <= end; i++) 
         frequency[arr[i]]++;    
  
     // Count elements with same frequency
     // as value
     int count = 0;
     for ( auto x : frequency) 
         if (x.first == x.second) 
             count++;    
     return count;
}
  
int main()
{
     int A[] = { 1, 2, 2, 3, 3, 3 };
     int n = sizeof (A) / sizeof (A[0]);
  
     // 2D array of queries with 2 columns
     int queries[][3] = { { 0, 1 }, { 1, 1 }, { 0, 2 }, { 1, 3 }, { 3, 5 }, { 0, 5 } };
  
     // calculating number of queries
     int q = sizeof (queries) / sizeof (queries[0]);
  
     for ( int i = 0; i < q; i++) {
         int start = queries[i][0];
         int end = queries[i][1];
         cout << "Answer for Query " << (i + 1)
              << " = " << solveQuery(start, end, A) << endl;
     }
  
     return 0;
}

Java

/* Java Program to answer Q queries to 
find number of times an element x 
appears x times in a Query subarray */
import java.util.HashMap;
import java.util.Map;
  
class GFG 
{
  
  
/* Returns the count of number x with
frequency x in the subarray from 
start to end */
static int solveQuery( int start, int end, int arr[])
{
     // map for frequency of elements
     Map<Integer, Integer> mp = new HashMap<>();
  
     // store frequency of each element 
     // in arr[start; end]
     for ( int i = start; i <= end; i++) 
         mp.put(arr[i], mp.get(arr[i]) == null ? 1 :mp.get(arr[i])+ 1 ); 
  
     // Count elements with same frequency
     // as value
     int count = 0 ;
     for (Map.Entry<Integer, Integer> entry : mp.entrySet()) 
         if (entry.getKey() == entry.getValue()) 
             count++; 
     return count;
}
  
// Driver code
public static void main(String[] args) 
{
     int A[] = { 1 , 2 , 2 , 3 , 3 , 3 };
     int n = A.length;
  
     // 2D array of queries with 2 columns
     int [][]queries = { { 0 , 1 }, { 1 , 1 }, { 0 , 2 }, { 1 , 3 }, { 3 , 5 }, { 0 , 5 } };
  
     // calculating number of queries
     int q = queries.length;
  
     for ( int i = 0 ; i < q; i++) 
     {
         int start = queries[i][ 0 ];
         int end = queries[i][ 1 ];
         System.out.println( "Answer for Query " + (i + 1 )
             + " = " + solveQuery(start, end, A));
     }
}
}
  
// This code is contributed by Rajput-Ji

Python3

# Python 3 Program to answer Q queries 
# to find number of times an element x 
# appears x times in a Query subarray
import math as mt
  
# Returns the count of number x with
# frequency x in the subarray from 
# start to end
def solveQuery(start, end, arr):
  
     # map for frequency of elements
     frequency = dict ()
  
     # store frequency of each element 
     # in arr[start end]
     for i in range (start, end + 1 ):
  
  
         if arr[i] in frequency.keys():
             frequency[arr[i]] + = 1
         else :
             frequency[arr[i]] = 1
                  
     # Count elements with same 
     # frequency as value
     count = 0
     for x in frequency: 
         if x = = frequency[x]: 
             count + = 1
     return count
  
# Driver code 
A = [ 1 , 2 , 2 , 3 , 3 , 3 ]
n = len (A)
  
# 2D array of queries with 2 columns
queries = [[ 0 , 1 ], [ 1 , 1 ], [ 0 , 2 ], [ 1 , 3 ], [ 3 , 5 ], [ 0 , 5 ]]
  
# calculating number of queries
q = len (queries)
  
for i in range (q):
     start = queries[i][ 0 ]
     end = queries[i][ 1 ]
     print ( "Answer for Query " , (i + 1 ), " = " , solveQuery(start, end, A))
            
# This code is contributed 
# by Mohit kumar 29

C#

// C# Program to answer Q queries to 
// find number of times an element x 
// appears x times in a Query subarray
using System;
using System.Collections.Generic;
  
class GFG
{
     /* Returns the count of number x with 
     frequency x in the subarray from 
     start to end */
     public static int solveQuery( int start, int end, int [] arr)
     {
  
         // map for frequency of elements 
         Dictionary< int , int > mp = new Dictionary< int , int >();
  
         // store frequency of each element 
         // in arr[start; end] 
         for ( int i = start; i <= end; i++)
         {
             if (mp.ContainsKey(arr[i]))
                 mp[arr[i]]++;
             else
                 mp.Add(arr[i], 1);
         }
  
         // Count elements with same frequency 
         // as value 
         int count = 0;
         foreach (KeyValuePair< int , int > entry in mp)
         {
             if (entry.Key == entry.Value)
                 count++;
         }
         return count;
     }
  
     // Driver code 
     public static void Main(String[] args)
     {
         int [] A = { 1, 2, 2, 3, 3, 3 };
         int n = A.Length;
  
         // 2D array of queries with 2 columns 
         int [, ] queries = {{ 0, 1 }, { 1, 1 }, { 0, 2 }, { 1, 3 }, { 3, 5 }, { 0, 5 }};
  
         // calculating number of queries 
         int q = queries.Length;
  
         for ( int i = 0; i < q; i++)
         {
             int start = queries[i, 0];
             int end = queries[i, 1];
             Console.WriteLine( "Answer for Query " + (i + 1) +
                               " = " + solveQuery(start, end, A));
         }
     }
}
  
// This code is contributed by
// sanjeev2552

输出如下：

Answer for Query 1 = 1
Answer for Query 2 = 0
Answer for Query 3 = 2
Answer for Query 4 = 1
Answer for Query 5 = 1
Answer for Query 6 = 3

该方法的时间复杂度为O(Q * N)

方法2(高效)

我们可以使用

MO的算法

.

我们为每个查询分配开始索引, 结束索引和查询编号, 每个查询采用以下形式：

起始索引(L)：查询涵盖的子数组的起始索引； Ending Index(R)：查询所涵盖的子数组的Ending Index；查询编号(索引)：由于查询已排序, 因此可以告诉我们查询的原始位置, 以便我们按原始顺序回答查询

首先, 我们将查询分为多个块, 然后使用自定义比较器对查询进行排序。

现在, 我们离线处理查询, 并保留两个指针, 即

MO_RIGHT

和

MO_LEFT

对于每个传入的查询, 我们将这些指针向前和向后移动, 并根据当前查询的开始和结束索引插入和删除元素。

让当前运行的答案为current_ans.

每当我们插入一个元素, 我们增加包含元素的频率, 如果这个频率等于我们刚包含的元素, 我们增加current_ans。如果这个元素的频率变成(当前元素+ 1), 这意味着这个元素被计入当current_ans等于其频率时, 因此在这种情况下我们需要减小current_ans。

每当我们删除/删除一个元素, 我们减小被排除元素的频率, 如果该频率等于我们刚排除的元素, 则增加current_ans。如果该元素的频率变为(current element – 1), 则意味着该元素被计入较早的位置当current_ans等于其频率时, 因此在这种情况下我们需要减小current_ans。

/* C++ Program to answer Q queries to
    find number of times an element x 
    appears x times in a Query subarray */
#include <bits/stdc++.h>
using namespace std;
  
// Variable to represent block size. 
// This is made global so compare() 
// of sort can use it.
int block;
  
// Structure to represent a query range
struct Query {
     int L, R, index;
};
  
/* Function used to sort all queries 
    so that all queries of same block
    are arranged together and within 
    a block, queries are sorted in 
    increasing order of R values. */
bool compare(Query x, Query y)
{
     // Different blocks, sort by block.
     if (x.L / block != y.L / block)
         return x.L / block < y.L / block;
  
     // Same block, sort by R value
     return x.R < y.R;
}
  
/* Inserts element (x) into current range
    and updates current answer */
void add( int x, int & currentAns, unordered_map< int , int >& freq)
{
  
     // increment frequency of this element
     freq[x]++;
  
     // if this element was previously 
     // contributing to the currentAns, // decrement currentAns
     if (freq[x] == (x + 1))
         currentAns--;
  
     // if this element has frequency 
     // equal to its value, increment
     // currentAns
     else if (freq[x] == x)
         currentAns++;
}
  
/* Removes element (x) from current 
    range btw L and R and updates 
    current Answer */
void remove ( int x, int & currentAns, unordered_map< int , int >& freq)
{
  
     // decrement frequency of this element
     freq[x]--;
  
     // if this element has frequency equal 
     // to its value, increment currentAns
     if (freq[x] == x)
         currentAns++;
  
     // if this element was previously 
     // contributing to the currentAns 
     // decrement currentAns
     else if (freq[x] == (x - 1)) 
         currentAns--;
}
  
/* Utility Function to answer all queries
    and build the ans array in the original 
    order of queries */
void queryResultsUtil( int a[], Query q[], int ans[], int m)
{
  
     // map to store freq of each element
     unordered_map< int , int > freq;
  
     // Initialize current L, current R
     // and current sum
     int currL = 0, currR = 0;
     int currentAns = 0;
  
     // Traverse through all queries
     for ( int i = 0; i < m; i++) {
         // L and R values of current range
         int L = q[i].L, R = q[i].R; 
         int index = q[i].index;
  
         // Remove extra elements of previous
         // range. For example if previous 
         // range is [0, 3] and current range 
         // is [2, 5], then a[0] and a[1] are 
         // removed
         while (currL < L) {
             remove (a[currL], currentAns, freq);
             currL++;
         }
  
         // Add Elements of current Range
         while (currL > L) {
             currL--;
             add(a[currL], currentAns, freq);
         }
         while (currR <= R) {
             add(a[currR], currentAns, freq);
             currR++;
         }
  
         // Remove elements of previous range.  For example
         // when previous range is [0, 10] and current range
         // is [3, 8], then a[9] and a[10] are Removed
         while (currR > R + 1) {
             currR--;
             remove (a[currR], currentAns, freq);
         }
  
         // Store current ans as the Query ans for
         // Query number index
         ans[index] = currentAns;
     }
}
  
/* Wrapper for queryResultsUtil() and outputs the
    ans array constructed by answering all queries */
void queryResults( int a[], int n, Query q[], int m)
{
     // Find block size
     block = ( int ) sqrt (n);
  
     // Sort all queries so that queries of same blocks
     // are arranged together.
     sort(q, q + m, compare);
  
     int * ans = new int [m];
     queryResultsUtil(a, q, ans, m);
  
     for ( int i = 0; i < m; i++) {
         cout << "Answer for Query " << (i + 1)
              << " = " << ans[i] << endl;
     }
}
  
// Driver program
int main()
{
     int A[] = { 1, 2, 2, 3, 3, 3 };
  
     int n = sizeof (A) / sizeof (A[0]);
  
     // 2D array of queries with 2 columns
     Query queries[] = { { 0, 1, 0 }, { 1, 1, 1 }, { 0, 2, 2 }, { 1, 3, 3 }, { 3, 5, 4 }, { 0, 5, 5 } };
  
     // calculating number of queries
     int q = sizeof (queries) / sizeof (queries[0]);
  
     // Print result for each Query
     queryResults(A, n, queries, q);
  
     return 0;
}

输出如下：

Answer for Query 1 = 1
Answer for Query 2 = 0
Answer for Query 3 = 2
Answer for Query 4 = 1
Answer for Query 5 = 1
Answer for Query 6 = 3

使用MO算法的这种方法的时间复杂度是O(Q * sqrt(N)* logA)其中logA是为每个查询将元素A插入unordered_map中的复杂度。

木子山

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