Crossword Formation


You’re a crossword fanatic, and have finally decided to try and create your own. However, you also love symmetry and good design, so you come up with a set of rules they should follow:

  • the crossword must contain exactly four words;
  • these four words should form four pairwise intersections;
  • all words must be written either left-to-right or top-to-bottom;
  • the area of the rectangle formed by empty cells inside the intersections isn’t equal to zero.

Given 4 words, find the number of ways to make a crossword following the above-described rules. Note that two crosswords which differ by rotation are considered different.


For words = ["crossword", "square", "formation", "something"], the output should be
crosswordFormation(words) = 6.

The six crosswords can be formed as shown below:

  f                         f                             f
  o                     c r o s s w o r d     c r o s s w o r d
c r o s s w o r d           r   o                   q     r
  m   q                     m   m                   u     m
  a   u            ;  s q u a r e          ;        a     a
  t   a                     t   t                   r     t
  i   r                     i   h             s o m e t h i n g
s o m e t h i n g           o   i                         o
  n                         n   n                         n
    c         s               s                                      
f o r m a t i o n       c     q               c         s          
    o         m         r     u               r         o      
    s q u a r e       f o r m a t i o n       o         m            
    s         t    ;    s     r            ;  s q u a r e                  
    w         h         s o m e t i n g       s         t         
    o         i         w                     w         h     
    r         n         o                   f o r m a t i o n            
    d         g         r                     r         n         
                        d                     d         g             


  • [time limit] 20000ms (swift)
  • [input] array.string words

    An array of distinct strings, the words you need to use in your crossword.

    words.length = 4,
    3 ≤ words[i].length < 15.

  • [output] integer

    The number of ways to make a correct crossword of the desired formation.


func crosswordFormation(words: [String]) -> Int {
    var count = 0
    for i in 0..<4 {
        let a = words[i]{String($0)})
        for a1 in 0..<(a.count - 1) {
            for j in 0..<4 {
                if j == i {
                let b = words[j]{String($0)})
                for b2 in 1..<b.count {
                    if b[b2] != a[a1] {
                    for b1 in 0..<(b2 - 1) {
                        for k in 0..<4 {
                            if k == i || k == j {
                            let c = words[k]{String($0)})
                            let d = words[6 - i - j - k]{String($0)})
                            if b2 - b1 >= d.count {
                            for c1 in 0..<(c.count - 1) {
                                if c[c1] != b[b1] {
                                for c2 in (c1 + 2)..<c.count {
                                    let a2 = a1 + (c2 - c1)
                                    if a2 >= a.count {
                                    for d1 in 0..<d.count {
                                        if d[d1] != c[c2] {
                                        let d2 = d1 + (b2 - b1)
                                        if d2 >= d.count {
                                        if a[a2] != d[d2] {
                                        count += 1
    return count