Array 的所有后缀的分解值总和

给定一个数组arr[] ，任务是找到后缀子数组的分解值之和。
分解值：子数组的分解值是子数组中可能的分区数。数组中索引处的分区$i$只有当数组的元素小于当前索引时才可以完成。即 A[k] < A[i]，其中 k ≤ i。
例子：

Input: arr[] = {2, 8, 4} 
Output: 4 
Explanation: 
All suffixes subarray of arr[] are [2, 8, 4], [8, 4], [4] 
Suffix [4] => only 1 decomposition {4} 
Suffix [8, 4] => only 1 decomposition {8, 4} 
Suffix [2, 8, 4] => 2 decompositions {2, 8, 4}, {2} {8, 4} 
Hence, Sum of Decomposition values = 1 + 1 + 2 = 4
Input: arr[] = {9, 6, 9, 35} 
Output: 8 
Explanation: 
All suffixes of arr are [9, 6, 9, 35], [6, 9, 35], [9, 35], [35] 
Suffix [35] => only 1 decomposition {35} 
Suffix [9, 35] => 2 decompositions {9} {35} 
Suffix [6, 9, 35] => 3 decompositions {6} {9, 35} 
Suffix [9, 6, 9, 35] => 2 decompositions {9, 6, 9} {35} 
Hence, Sum of Decomposition values = 1 + 2 + 3 + 2 = 8 
 

做法：思路是用Stack来解决这个问题。下面是该方法的说明

• 从头到尾遍历数组。
• 保持最小变量和答案变量。
• 如果堆栈为空或当前元素小于堆栈顶部 –
  • 将 S[i] 压入堆栈。
  • 按堆栈的大小增加答案。
  • 另外，保持最小值直到现在。
• 除此以外，
  • 只要栈顶小于当前元素，就继续弹出块。
  • 用当前元素更新到现在的最小值。
  • 将最小值压入堆栈。因为，我们希望子数组的最小值来表示该子数组
  • 按堆栈的大小增加答案。

下面是上述方法的实现：

C++

// C++ implementation to find the
// sum of Decomposition values of
// all suffixes of an array
 
#include 
using namespace std;
#define int long long int
 
// Function to find the decomposition
// values of the array
int decompose(vector S)
{
    // Stack
    stack s;
    int N = S.size();
    int ans = 0;
     
    // Variable to maintain
    // min value in stack
    int nix = INT_MAX;
     
    // Loop to iterate over the array
    for (int i = N - 1; i >= 0; i--) {
         
        // Condition to check if the
        // stack is empty
        if (s.empty()) {
            s.push(S[i]);
            nix = S[i];
        }
        else {
             
            // Condition to check if the
            // top of the stack is greater
            // than the current element
            if (S[i] < s.top()) {
                s.push(S[i]);
                nix = min(nix, S[i]);
            }
            else {
                int val = S[i];
                 
                // Loop to pop the element out
                while (!s.empty() &&
                       val >= s.top()) {
                    s.pop();
                }
                nix = min(nix, S[i]);
                s.push(nix);
            }
        }
         
        // the size of the stack is the
        // max no of subarrays for
        // suffix till index i
        // from the right
        ans += s.size();
    }
 
    return ans;
}
 
// Driver Code
signed main()
{
    vector S = { 9, 6, 9, 35 };
    cout << decompose(S) << endl;
    return 0;
}

Java

// Java implementation to find the
// sum of Decomposition values of
// all suffixes of an array
import java.util.*;
 
class GFG{
 
// Function to find the decomposition
// values of the array
static int decompose(Vector S)
{
     
    // Stack
    Stack s = new Stack();
    int N = S.size();
    int ans = 0;
     
    // Variable to maintain
    // min value in stack
    int nix = Integer.MAX_VALUE;
     
    // Loop to iterate over the array
    for(int i = N - 1; i >= 0; i--)
    {
         
       // Condition to check if the
       // stack is empty
       if (s.isEmpty())
       {
           s.add(S.get(i));
           nix = S.get(i);
       }
       else
       {
            
           // Condition to check if the
           // top of the stack is greater
           // than the current element
           if (S.get(i) < s.peek())
           {
               s.add(S.get(i));
               nix = Math.min(nix, S.get(i));
           }
           else
           {
               int val = S.get(i);
                
               // Loop to pop the element out
               while (!s.isEmpty() && val >= s.peek())
               {
                   s.pop();
               }
               nix = Math.min(nix, S.get(i));
               s.add(nix);
           }
       }
        
       // The size of the stack is the
       // max no of subarrays for
       // suffix till index i
       // from the right
       ans += s.size();
    }
    return ans;
}
 
// Driver Code
public static void main(String args[])
{
    Vector S = new Vector();
    S.add(9);
    S.add(6);
    S.add(9);
    S.add(35);
     
    System.out.println(decompose(S));
}
}
 
// This code is contributed by 29AjayKumar

Python3

# Python3 implementation to find the
# sum of Decomposition values of
# all suffixes of an array
import sys
 
# Function to find the decomposition
# values of the array
def decompose(S):
 
    # Stack
    s = []
    N = len(S)
    ans = 0
     
    # Variable to maintain
    # min value in stack
    nix = sys.maxsize
     
    # Loop to iterate over the array
    for i in range(N - 1, -1, -1):
         
        # Condition to check if the
        # stack is empty
        if (len(s) == 0):
            s.append(S[i])
            nix = S[i]
         
        else:
             
            # Condition to check if the
            # top of the stack is greater
            # than the current element
            if (S[i] < s[-1]):
                s.append(S[i])
                nix = min(nix, S[i])
             
            else:
                val = S[i]
                 
                # Loop to pop the element out
                while (len(s) != 0 and
                          val >= s[-1]):
                    s.pop()
             
                nix = min(nix, S[i]);
                s.append(nix)
         
        # The size of the stack is the
        # max no of subarrays for
        # suffix till index i
        # from the right
        ans += len(s)
 
    return ans
 
# Driver Code
if __name__ =="__main__":
     
    S = [ 9, 6, 9, 35 ]
     
    print(decompose(S))
 
# This code is contributed by chitranayal

C#

// C# implementation to find the
// sum of Decomposition values of
// all suffixes of an array
using System;
using System.Collections.Generic;
 
class GFG{
 
// Function to find the decomposition
// values of the array
static int decompose(List S)
{
     
    // Stack
    Stack s = new Stack();
     
    int N = S.Count;
    int ans = 0;
     
    // Variable to maintain
    // min value in stack
    int nix = Int32.MaxValue;
     
    // Loop to iterate over the array
    for(int i = N - 1; i >= 0; i--)
    {
         
        // Condition to check if the
        // stack is empty
        if (s.Count == 0)
        {
            s.Push(S[i]);
            nix = S[i];
        }
        else
        {
             
            // Condition to check if the
            // top of the stack is greater
            // than the current element
            if (S[i] < s.Peek())
            {
                s.Push(S[i]);
                nix = Math.Min(nix, S[i]);
            }
            else
            {
                int val = S[i];
                     
                // Loop to pop the element out
                while (s.Count != 0 && val >= s.Peek())
                {
                    s.Pop();
                }
                nix = Math.Min(nix, S[i]);
                s.Push(nix);
            }
        }
         
        // The size of the stack is the
        // max no of subarrays for
        // suffix till index i
        // from the right
        ans += s.Count;
    }
    return ans;
}
 
// Driver code
static void Main()
{
    List S = new List();
    S.Add(9);
    S.Add(6);
    S.Add(9);
    S.Add(35);
 
    Console.WriteLine(decompose(S));
}
}
 
// This code is contributed by divyeshrabadiya07

Javascript

// Javascript implementation to find the
// sum of Decomposition values of
// all suffixes of an array
 
// Function to find the decomposition
// values of the array
function decompose(S)
{
 
    // Stack
    let s = [];
    let N = S.length;
    let ans = 0;
       
    // Variable to maintain
    // min value in stack
    let nix = Number.MAX_VALUE;
       
    // Loop to iterate over the array
    for(let i = N - 1; i >= 0; i--)
    {
           
       // Condition to check if the
       // stack is empty
       if (s.length==0)
       {
           s.push(S[i]);
           nix = S[i];
       }
       else
       {
              
           // Condition to check if the
           // top of the stack is greater
           // than the current element
           if (S[i] < s[s.length-1])
           {
               s.push(S[i]);
               nix = Math.min(nix, S[i]);
           }
           else
           {
               let val = S[i];
                  
               // Loop to pop the element out
               while (s.length!=0 && val >= s[s.length-1])
               {
                   s.pop();
               }
               nix = Math.min(nix, S[i]);
               s.push(nix);
           }
       }
          
       // The size of the stack is the
       // max no of subarrays for
       // suffix till index i
       // from the right
       ans += s.length;
    }
    return ans;
}
 
// Driver Code
let S = [];
S.push(9);
S.push(6);
S.push(9);
S.push(35);
 
document.write(decompose(S));
 
// This code is contributed by avanitrachhadiya2155

输出：

8

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