766A - Mahmoud and Longest Uncommon Subsequence

If the strings are the same, Any subsequence of a is indeed a subsequence of b so the answer is "-1", Otherwise the longer string can't be a subsequence of the other (If they are equal in length and aren't the same, No one can be a subsequence of the other) so the answer is maximum of their lengths.

Code : http://pastebin.com/aJbeTTjw

Time complexity : O(|a| + |b|).

Problem author : me.

Solution author : me.

Testers : me and mahmoudbadawy.

766B - Mahmoud and a Triangle

First solution :-

Let x, y and z be the lengths of 3 line segments such that x ≤ y ≤ z, If they can't form a non-degenerate triangle, Line segments of lengths x - 1, y and z or x, y and z + 1 can't form a non-degenerate triangle, So we don't need to try all the combinations, If we try y as the middle one, We need to try the maximum x that is less than or equal to y and the minimum z that is greater than or equal to y, The easiest way to do so is to sort the line segments and try every consecutive 3.

Code : http://pastebin.com/NsCkbQFS

Time complexity : O(nlog(n)).

Second solution :-

Depending on the note from the first solution, If we try to generate a sequence such that after sorting, Every consecutive 3 line segments will form a degenerate triangle, It will be 1 1 2 3 5 8 13 ... which is Fibonacci sequence, Fibonacci is a fast growing sequence, fib(45) = 1134903170, Notice that Fibonacci makes maximum n with "NO" as the answer, That means the answer is indeed "YES" for n ≥ 45, For n < 45, You can do the naive O(n³) solution or the first solution.

Code : http://pastebin.com/82XcJfgp

Let x be the number that satisfies these inequalities:-

fib(x) ≤ maxAi.

fib(x + 1) > maxAi.

Time complexity : O(x³) or O(xlog(x)).

Problem author : me.

Solutions author : me.

Testers : me and mahmoudbadawy.

766C - Mahmoud and a Message

Let dp[i] be the number of ways to split the prefix of s ending at index i into substrings that fulfills the conditions. Let it be 1-indexed. Our base case is dp[0] = 1. Our answer is dp[n]. Now let's calculate it for every i. Let l be the minimum possible index such that the substring from l to i satisfies the condition, Let x be a moving pointer, At the beginning x = i - 1 and it decreases, Every time we decrease x, We calculate the new value of l depending on the current character like that, l = max(l, i - a_s[x]). While x is greater than or equal to l we add dp[x] to dp[i], To find the longest substring, Find maximum i - x, To find the minimum number of substrings, there is an easy greedy solution, Find the longest valid prefix and delete it and do the same again until the string is empty, The number of times this operation is repeated is our answer, Or see the dynamic programming solution in the code.

Code : http://pastebin.com/4JiXSwfU

Time complexity : O(n²).

Try to find an O(n) solution(I'll post a hard version of some problems on this blog soon).

Problem authors : me and mahmoudbadawy.

Solution authors : me and mahmoudbadawy.

Testers : me and mahmoudbadawy.

766D - Mahmoud and a Dictionary

Let's build a graph containing the words, For every relation in the input add a new edge with the weight of 0 if they are equal and 1 if they are opposites, If adding the edge doesn't make the graph cyclic, Our relation is valid, Otherwise it may be valid or invalid so we'll answer them offline. Check if adding that edge will make the graph cyclic or not using union-find like Kruskal's algorithm. Suspend answering relations that will make the graph cyclic, Now we have a forest of trees, Let cum[i] be the xor of the weights on the edges in the path from the root of the component of node i to node i. Calculate it using dfs. To find the relation between 2 words u and v, Check if they are in the same component using union-find, If they aren't, The answer is 3 otherwise the answer is , Now to answer suspended relations, Find the relation between the 2 words and check if it's the same as the input relation, Then answer the queries.

Code : http://pastebin.com/WqwduaYs

Time complexity : O((n + m + q)log(n) * maxL) where maxL is the length of the longest string considering that union-find works in constant time.

Problem author : mahmoudbadawy.

Solution author : me.

Testers : me and mahmoudbadawy.

Wait for a hard version of this problem.

766E - Mahmoud and a xor trip

If we have an array ans[i] which represents the number of paths that makes the i^th bit sit to 1, Our answer will be

Let arr[i][x] be the binary value of the x^th bit of the number attached to node i(just to make work easier).

There are 2 types of paths from node u to node v where u is less in depth than or equal to v, Paths going down which are paths with lca(u, v)=u and other paths, Let's root the tree at node 1 and dfs, let current node be node, Let dp[i][x][j] be the number of paths going down from node i that makes the x^th bit's value equal to j. A path going down from node i is a path going down from a child of i with node i concatenated to it so let's calculate our dp. A path that isn't going down is a concatenation of 2 paths which are going down from lca(u, v), Now we can calculate ans. See the code for formulas.

Code : http://pastebin.com/n2a3kijD

Time complexity : O(nlog(a_i)).

Problem author : me.

Solution author : me.

Tester : mahmoudbadawy.