2SUM

Solution to Rosalind Algorithmic Heights Problem 008: 2SUM

Algorithmic_008_2SUM.py

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+#!/usr/bin/env python
+'''
+A solution to a ROSALIND problem from the Algorithmic Heights problem area.
+Algorithmic Heights focuses on teaching algorithms and data structures commonly used in computer science.
+
+Problem Title: 2SUM
+Rosalind ID: 2SUM
+Algorithmic Heights #: 008
+URL: http://rosalind.info/problems/2sum/
+'''
+
+from collections import defaultdict
+
+
+def two_sum(a, x):
+    '''Returns indices i and j such that a[i] + a[j] = x if they exist, otherwise -1.'''
+    # Hash the array, storing the indices corresponding to each element.
+    elmt_hash = defaultdict(set)
+    for index, e in enumerate(a):
+        elmt_hash[e].add(index)
+
+    # Loop over all elements in the hash, checking for the necessary element to complete 2sum.
+    for e in elmt_hash:
+        if x-e in elmt_hash:
+            if x-e == e and len(elmt_hash[e]) > 1:
+                # Rosalind indices start at 1.
+                return sorted([elmt_hash[e].pop()+1, elmt_hash[e].pop()+1])
+            elif x-e != e:
+                # Rosalind indices start at 1.
+                return sorted([elmt_hash[e].pop()+1, elmt_hash[x-e].pop()+1])
+
+    # If the loop ends 2sum is not possible.
+    return -1
+
+
+def main():
+    '''Main call. Reads, runs, and saves problem specific data.'''
+    # Read the input data.
+    with open('data/algorithmic/rosalind_2sum.txt') as input_data:
+        k, n = map(int, input_data.readline().strip().split())
+        arrays = [map(int, line.strip().split()) for line in input_data.readlines()]
+
+    # Run the 2sum algorithm on each array, then format the results.
+    indices_2sum = [two_sum(a, 0) for a in arrays]
+    indices_2sum = [str(x) if type(x) is int else ' '.join(map(str, x)) for x in indices_2sum]
+
+    # Print and save the answer.
+    print '\n'.join(indices_2sum)
+    with open('output/algorithmic/Algorithmic_008_2SUM.txt', 'w') as output_data:
+        output_data.write('\n'.join(indices_2sum))
+
+if __name__ == '__main__':
+    main()

output/algorithmic/Algorithmic_008_2SUM.txt

@@ -0,0 +1,15 @@
+3378 5857
+3962 7424
+6723 9229
+8675 9435
+2124 2580
+6576 8386
+3364 6142
+5176 9648
+6762 8689
+1491 3826
+6437 6590
+4063 5870
+-1
+2375 6311
+4255 4639