Finding the Number of Common Letters in an Array of Strings

def common(arr):
    L1 = []

    for i in range(len(arr)-1):
        str1 = arr[i]

        for j in str1:
            str2 = arr[i+1]

            if str2.find(j) >= 0:
                if j not in L1:
                    L1.append(j)

    i = 0
    found = True

    while i < len(L1):
        j = 0

        while j < len(arr):
            if (arr[j].find(L1[i])) < 0:
                found = False
                break

            elif (arr[j].find(L1[i])) >= 0:
                found = True

            j += 1

        if not found:
            L1.pop(i)
            i = 0
        else:
            i += 1

        found = True
        continue

    return len(L1)

val = common(['abc', 'bac', 'ab'])
print(val)