Power Set in Lexicographic order

This article is about generating Power set in lexicographical order.

Examples :

Input : abc
Output : a ab abc ac b bc c


The idea is to sort array first. After sorting, one by one fix characters and recursively generates all subsets starting from them. After every recursive call, we remove last character so that next permutation can be generated.

C++

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// CPP program to generate power set in
// lexicographic order.
#include <bits/stdc++.h>
using namespace std;
  
// str : Stores input string
// n : Length of str.
// curr : Stores current permutation
// index : Index in current permutation, curr
void permuteRec(string str, int n,
           int index = -1, string curr = "")
{
    // base case
    if (index == n) 
        return;
  
    cout << curr << "\n";
    for (int i = index + 1; i < n; i++) {
  
        curr += str[i];
        permuteRec(str, n, i, curr);
   
        // backtracking
        curr = curr.erase(curr.size() - 1); 
    }
    return;
}
  
// Generates power set in lexicographic
// order.
void powerSet(string str)
{
    sort(str.begin(), str.end());
    permuteRec(str, str.size());
}
  
// Driver code
int main()
{
    string str = "cab";
    powerSet(str);
    return 0;
}

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PHP

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<?php
// PHP program to generate power 
// set in lexicographic order.
  
// str : Stores input string
// n : Length of str.
// curr : Stores current permutation
// index : Index in current permutation, curr
function permuteRec($str, $n, $index = -1, 
                              $curr = "")
{
    // base case
    if ($index == $n
        return;
  
    echo $curr."\n";
    for ($i = $index + 1; $i < $n; $i++) 
    {
  
        $curr=$curr.$str[$i];
        permuteRec($str, $n, $i, $curr);
  
        // backtracking
        $curr =""
    }
    return;
}
  
// Generates power set in lexicographic
// order.
function powerSet($str)
{
  
    $str = str_split($str);
    sort($str);
    permuteRec($str, sizeof($str));
}
  
// Driver code
$str = "cab";
powerSet($str);
  
// This code is contributed by Mithun Kumar
?>

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Output :

a
ab
abc
ac
b
bc
c


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Improved By : Mithun Kumar