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.

Examples:

Input:[10, 22, 9, 33, 21, 50, 41, 60, 80]Output:[10, 22, 33, 50, 60, 80] OR [10 22 33 41 60 80] or any other LIS of same length.

In previous post, we have discussed about Longest Increasing Subsequence problem. However, the post only covered code related to querying size of LIS, but not the construction of LIS. In this post, we will discuss how to print LIS using similar DP solution discussed earlier.

Let arr[0..n-1] be the input array. We define vector L such that L[i] is itself is a vector that stores LIS of arr that ends with arr[i]. For example, for array [3, 2, 6, 4, 5, 1],

L[0]: 3 L[1]: 2 L[2]: 2 6 L[3]: 2 4 L[4]: 2 4 5 L[5]: 1

Therefore for an index i, L[i] can be recursively written as –

L[0] = {arr[O]} L[i] = {Max(L[j])} + arr[i] where j < i and arr[j] < arr[i] and if there is no such j then L[i] = arr[i]

Below is C++ implementation of above idea –

`/* Dynamic Programming solution to construct Longest ` ` ` `Increasing Subsequence */` `#include <iostream> ` `#include <vector> ` `using` `namespace` `std; ` ` ` `// Utility function to print LIS ` `void` `printLIS(vector<` `int` `>& arr) ` `{ ` ` ` `for` `(` `int` `x : arr) ` ` ` `cout << x << ` `" "` `; ` ` ` `cout << endl; ` `} ` ` ` `// Function to construct and print Longest Increasing ` `// Subsequence ` `void` `constructPrintLIS(` `int` `arr[], ` `int` `n) ` `{ ` ` ` `// L[i] - The longest increasing sub-sequence ` ` ` `// ends with arr[i] ` ` ` `vector<vector<` `int` `> > L(n); ` ` ` ` ` `// L[0] is equal to arr[0] ` ` ` `L[0].push_back(arr[0]); ` ` ` ` ` `// start from index 1 ` ` ` `for` `(` `int` `i = 1; i < n; i++) ` ` ` `{ ` ` ` `// do for every j less than i ` ` ` `for` `(` `int` `j = 0; j < i; j++) ` ` ` `{ ` ` ` `/* L[i] = {Max(L[j])} + arr[i] ` ` ` `where j < i and arr[j] < arr[i] */` ` ` `if` `((arr[i] > arr[j]) && ` ` ` `(L[i].size() < L[j].size() + 1)) ` ` ` `L[i] = L[j]; ` ` ` `} ` ` ` ` ` `// L[i] ends with arr[i] ` ` ` `L[i].push_back(arr[i]); ` ` ` `} ` ` ` ` ` `// L[i] now stores increasing sub-sequence of ` ` ` `// arr[0..i] that ends with arr[i] ` ` ` `vector<` `int` `> max = L[0]; ` ` ` ` ` `// LIS will be max of all increasing sub- ` ` ` `// sequences of arr ` ` ` `for` `(vector<` `int` `> x : L) ` ` ` `if` `(x.size() > max.size()) ` ` ` `max = x; ` ` ` ` ` `// max will contain LIS ` ` ` `printLIS(max); ` `} ` ` ` `// Driver function ` `int` `main() ` `{ ` ` ` `int` `arr[] = { 3, 2, 6, 4, 5, 1 }; ` ` ` `int` `n = ` `sizeof` `(arr) / ` `sizeof` `(arr[0]); ` ` ` ` ` `// construct and print LIS of arr ` ` ` `constructPrintLIS(arr, n); ` ` ` ` ` `return` `0; ` `} ` |

Output:

2 4 5

Note that the time complexity of the above Dynamic Programming (DP) solution is O(n^2) and there is a O(n Log n) non-DP solution for the LIS problem. See below post for O(n Log n) solution.

Construction of Longest Monotonically Increasing Subsequence (N log N)

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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