Subset sum problem where Array sum is at most N
Last Updated :
16 Feb, 2023
Given an array arr[] of size N such that the sum of all the array elements does not exceed N, and array queries[] containing Q queries. For each query, the task is to find if there is a subset of the array whose sum is the same as queries[i].
Examples:
Input: arr[] = {1, 0, 0, 0, 0, 2, 3}, queries[] = {3, 7, 6}
Output:
Possible
Not Possible
Possible
Explanation: 3 is spossible. 6 can be obtained by the subset {1, 2, 3}
7 is greater than the sum of all array elements.
Input: arr[] = {0, 1, 2}, queries[] = {1, 2, 3, 0}
Output:
Possible
Possible
Possible
Possible
Explanation: All the sums can be obtained by using the elements.
Approach: The problem can be solved using the approach as in the subset sum problem.
However, the time complexity can be reduced using the fact that the sum can be at most N. As the sum can be at most N, it can be proved that there are at most √2N unique positive elements where all have a frequency of 1.
Say there are √2N unique positive elements starting from 1 to √2N.
Therefore the sum of those numbers is N + √(N/2).
This sum is more than N itself. So there can be at most √2N unique elements.
The above fact can be used and implemented in dynamic programming. Using coordinate compression all those unique elements can be stored in minimum space.
For each element check what is the minimum contribution of that element to achieve a sum j (j in the range [0, N]) or if it is not possible to achieve the sum j. The contribution of each item (say i) depends on the contribution of the other smaller items till the sum (j – i).
Follow the image shown below to understand better the difference of unused states for normal subset and when the sum is N at max:
Comparison:
Say the arr[] = {1, 2, 2, 2, 3, 3}. (Here sum is greater, so does not follow the condition of sum at most N. But here unique elements maintain the threshold. That’s why it is used here just for understanding purpose)
Red cells signify the useless states, these are much more in traditional algorithm than optimized one.
Traditional Subset-Sum vs Frequency Optimized DP, Useless States
Follow the steps mentioned below to implement the approach;
- Use coordinate compression on all the unique elements.
- Build a 2D dp[][] array where dp[i][j] stores the contribution of ith item to get sum j. (If it is not possible then store –1, and if ith item is not needed then store 0 in dp[i][j]).
- Iterate from i = 0 to the maximum element:
- Iterate for j = 0 to N:
- If the value of dp[i][j-arr[i]] + 1 < dp[i][j] then update it.
- Otherwise, keep it as it was.
- Then iterate from i = 0 to Q:
- Check if that sum (query[i])is possible or not.
- It is not possible if it exceeds the array sum or all the elements together cannot get a certain sum i.e. dp[len][query[i]] = -1. (len is total number of unique elements)
Below is the implementation of the above approach.
C++
#include <bits/stdc++.h>
using namespace std;
void findSol(vector<int>& arr,
vector<int>& queries)
{
int s = 0;
for (auto& item : arr) {
s += item;
}
map<int, int> mp;
for (auto& item : arr) {
mp[item]++;
}
vector<int> val, freq;
for (auto& x : mp) {
val.push_back(x.first);
freq.push_back(x.second);
}
int len = val.size();
vector<vector<int> > dp(len + 1,
vector<int>(
s + 1, 0));
for (int j = 1; j <= s; ++j) {
dp[0][j] = -1;
}
for (int i = 1; i <= len; ++i) {
for (int j = 1; j <= s; ++j) {
int v = val[i - 1];
int f = freq[i - 1];
if (dp[i - 1][j] != -1) {
dp[i][j] = 0;
}
else if (j >= v
&& dp[i][j - v] != -1
&& dp[i][j - v] + 1 <= f) {
dp[i][j] = dp[i][j - v] + 1;
}
else {
dp[i][j] = -1;
}
}
}
for (auto& q : queries) {
if (q > s || dp[len][q] == -1) {
cout << "Not Possible" << endl;
}
else {
cout << "Possible" << endl;
}
}
}
int main()
{
vector<int> arr = { 1, 0, 0, 0, 0, 2, 3 };
vector<int> queries = { 3, 7, 6 };
findSol(arr, queries);
return 0;
}
|
Java
import java.util.*;
class GFG
{
static void findSol(ArrayList<Integer> arr, ArrayList<Integer> queries)
{
int s = 0;
for (Integer item : arr) {
s += item;
}
HashMap<Integer, Integer> mp = new HashMap<>();
for (Integer item : arr) {
if(mp.containsKey(item))
mp.put(item,mp.get(item)+1);
else
mp.put(item,1);
}
ArrayList<Integer> val = new ArrayList<Integer>();
ArrayList<Integer> freq = new ArrayList<Integer>();
for (Map.Entry<Integer,Integer> x : mp.entrySet())
{
val.add(x.getKey());
freq.add(x.getValue());
}
int len = val.size();
int dp[][] = new int[len+1][s+1];
for (int j = 1; j <= s; ++j) {
dp[0][j] = -1;
}
for (int i = 1; i <= len; ++i) {
for (int j = 1; j <= s; ++j) {
int v = val.get(i - 1);
int f = freq.get(i - 1);
if (dp[i - 1][j] != -1) {
dp[i][j] = 0;
}
else if (j >= v
&& dp[i][j - v] != -1
&& dp[i][j - v] + 1 <= f) {
dp[i][j] = dp[i][j - v] + 1;
}
else {
dp[i][j] = -1;
}
}
}
for(Integer q:queries)
{
if (q > s || dp[len][q] == -1) {
System.out.println("Not Possible");
}
else {
System.out.println("Possible");
}
}
}
public static void main(String[] args)
{
ArrayList<Integer> arr = new ArrayList<Integer>(
Arrays.asList(1, 0, 0, 0, 0, 2, 3 ));
ArrayList<Integer> queries = new ArrayList<Integer>(
Arrays.asList(3, 7, 6 ));
findSol(arr, queries);
}
}
|
Python3
def findSol(arr, queries):
s = sum(arr)
mp = dict()
for item in arr:
if item not in mp:
mp[item] = 1
else:
mp[item] += 1
val = []
freq = []
for x in mp:
val.append(x)
freq.append(mp[x])
len_ = len(val)
dp = [[0 for i in range(s + 1)] for j in range(len_ + 1)]
for j in range(1, s + 1):
dp[0][j] = -1
for i in range(1, len_ + 1):
for j in range(1, s + 1):
v = val[i - 1]
f = freq[i - 1]
if dp[i - 1][j] != -1:
dp[i][j] = 0
elif j >= 0 and dp[i][j - v] != -1 and dp[i][j - v] + 1 <= f:
dp[i][j] = dp[i][j - v] + 1
else:
dp[i][j] = -1
for q in queries:
if q > s or dp[len_][q] == -1:
print("Not Possible")
else:
print("Possible")
arr = [1, 0, 0, 0, 0, 2, 3]
queries = [3, 7, 6]
findSol(arr, queries)
|
C#
using System;
using System.Collections.Generic;
class GFG
{
public static void findSol(List<int> arr, List<int> queries)
{
int s = 0;
foreach (int item in arr){
s += item;
}
SortedDictionary<int, int> mp = new SortedDictionary<int, int>();
foreach (int item in arr) {
if(mp.ContainsKey(item)){
mp[item] = mp[item] + 1;
}else{
mp.Add(item, 1);
}
}
List<int> val = new List<int>();
List<int> freq = new List<int>();
foreach (KeyValuePair<int,int> x in mp)
{
val.Add(x.Key);
freq.Add(x.Value);
}
int len = val.Count;
int[,] dp = new int[len+1, s+1];
for (int j = 1; j <= s; ++j) {
dp[0, j] = -1;
}
for (int i = 1; i <= len; ++i) {
for (int j = 1; j <= s; ++j) {
int v = val[i - 1];
int f = freq[i - 1];
if (dp[i - 1, j] != -1) {
dp[i, j] = 0;
}
else if (j >= v && dp[i, j - v] != -1 && dp[i, j - v] + 1 <= f) {
dp[i, j] = dp[i, j - v] + 1;
}
else{
dp[i, j] = -1;
}
}
}
foreach(int q in queries)
{
if (q > s || dp[len, q] == -1) {
Console.Write("Not Possible\n");
}
else {
Console.Write("Possible\n");
}
}
}
public static void Main(string[] args){
List<int> arr = new List<int>{
1, 0, 0, 0, 0, 2, 3
};
List<int> queries = new List<int>{
3, 7, 6
};
findSol(arr, queries);
}
}
|
Javascript
<script>
function findSol(arr,queries)
{
let s = 0;
for (let item of arr) {
s += item;
}
let mp = new Map();
for (let item of arr) {
if(mp.has(item))
mp.set(item,mp.get(item)+1);
else mp.set(item,1);
}
let val = [], freq = [];
for (let [x,y] of mp) {
val.push(x);
freq.push(y);
}
let len = val.length;
let dp = new Array(len + 1).fill(0).map(()=>new Array(s+1).fill(0));
for (let j = 1; j <= s; ++j) {
dp[0][j] = -1;
}
for (let i = 1; i <= len; ++i) {
for (let j = 1; j <= s; ++j) {
let v = val[i - 1];
let f = freq[i - 1];
if (dp[i - 1][j] != -1) {
dp[i][j] = 0;
}
else if (j >= v
&& dp[i][j - v] != -1
&& dp[i][j - v] + 1 <= f) {
dp[i][j] = dp[i][j - v] + 1;
}
else {
dp[i][j] = -1;
}
}
}
for (let q of queries) {
if (q > s || dp[len][q] == -1) {
console.log("Not Possible");
}
else {
console.log("Possible");
}
}
}
let arr = [ 1, 0, 0, 0, 0, 2, 3 ];
let queries = [ 3, 7, 6 ];
findSol(arr, queries);
</script>
|
Output
Possible
Not Possible
Possible
Time Complexity: O(N * √N)
Auxiliary Space: O(N * √N)
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