C++ Program for Longest Increasing Subsequence

The Longest Increasing Subsequence (LIS) problem is to find the length of the longest subsequence of a given sequence such that all elements of the subsequence are sorted in increasing order. For example, the length of LIS for {10, 22, 9, 33, 21, 50, 41, 60, 80} is 6 and LIS is {10, 22, 33, 50, 60, 80}.
longest-increasing-subsequence

Examples:

Input  : arr[] = {3, 10, 2, 1, 20}
Output : Length of LIS = 3
The longest increasing subsequence is 3, 10, 20

Input  : arr[] = {3, 2}
Output : Length of LIS = 1
The longest increasing subsequences are {3} and {2}

Input : arr[] = {50, 3, 10, 7, 40, 80}
Output : Length of LIS = 4
The longest increasing subsequence is {3, 7, 40, 80}

Overlapping Subproblems:
Considering the above implementation, following is recursion tree for an array of size 4. lis(n) gives us the length of LIS for arr[].

              lis(4)
        /        |     
      lis(3)    lis(2)   lis(1)
     /           /
   lis(2) lis(1) lis(1)
   /
lis(1)

We can see that there are many subproblems which are solved again and again. So this problem has Overlapping Substructure property and recomputation of same subproblems can be avoided by either using Memoization or Tabulation. Following is a tabulated implementation for the LIS problem.

C++

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/* Dynamic Programming C/C++ implementation of LIS problem */
#include <stdio.h>
#include <stdlib.h>
  
/* lis() returns the length of the longest increasing
  subsequence in arr[] of size n */
int lis(int arr[], int n)
{
    int *lis, i, j, max = 0;
    lis = (int*)malloc(sizeof(int) * n);
  
    /* Initialize LIS values for all indexes */
    for (i = 0; i < n; i++)
        lis[i] = 1;
  
    /* Compute optimized LIS values in bottom up manner */
    for (i = 1; i < n; i++)
        for (j = 0; j < i; j++)
            if (arr[i] > arr[j] && lis[i] < lis[j] + 1)
                lis[i] = lis[j] + 1;
  
    /* Pick maximum of all LIS values */
    for (i = 0; i < n; i++)
        if (max < lis[i])
            max = lis[i];
  
    /* Free memory to avoid memory leak */
    free(lis);
  
    return max;
}
  
/* Driver program to test above function */
int main()
{
    int arr[] = { 10, 22, 9, 33, 21, 50, 41, 60 };
    int n = sizeof(arr) / sizeof(arr[0]);
    printf("Length of lis is %d\n", lis(arr, n));
    return 0;
}

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Python 3

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# Dyanmic Programming python3
# implementation of LIS problem
  
# lis() returns the length of the 
# longest increasing subsequence 
# in arr[] of size n
def lis(arr, n):
    i, j, maxm = 0, 0, 0
      
    # initialize LIS values for all indexes
    lst = [1 for s in range(n)]
      
    for i in range(1, n):
        for j in range(0, i):
            if (arr[i] > arr[j] and 
                lst[i] < lst[j] + 1):
                lst[i] = lst[j] + 1
      
    # Pick maximum of all LIS values
    for i in range(0, n):
        if maxm < lst[i]:
            maxm = lst[i]
      
    return maxm
  
# Driver Code
arr = [10, 22, 9, 33, 21, 50, 41, 60]
n = len(arr)
print("Length of lst is", lis(arr, n))
  
# This code is contributed 
# by Mohit kumar 29

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Output:

Length of lis is 5

Please refer complete article on Dynamic Programming | Set 3 (Longest Increasing Subsequence) for more details!



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Improved By : mohit kumar 29