https://codility.com/demo/results/trainingVV6DFE-KJ3/

oddoccurances.py

# you can write to stdout for debugging purposes, e.g.
# print "this is a debug message"

def solutionA(A): # 100% but space complexity is not O(1)
    L = len(A)
    d = {}
    
    if not L: return

    for e in A:
        if d.has_key(e):
            d.pop(e)
        else:
            d[e] = True
    return d.keys()[0]

def solutionB(A): # 77% (detected O(n^2) run time?? but space complexity O(1) due to quicksort
    # write your code in Python 2.7
    L = len(A)
    if L == 1:
        return A[0]
        
    qsort(A, 0, L)
    while i < L -1:
      while i < L-1 and A[i] == A[i+1]:
        count += 1
        i += 1
      if count % 2 == 0:
        count = 1
        i += 1
      else:
        break
    
    return A[i]


def qsort(A, L, U):
  if L < U:
    P = A[L]
    W = L
    for i in range(L, U):
      if A[i] < P:
        W += 1
        tmp = A[i]
        A[i] = A[W]
        A[W] = tmp
    A[L] = A[W]
    A[W] = P
    
    qsort(A, L, W)
    qsort(A, W+1, U)