first unique char in string

char? (s):
    if s == null:
        throw
    countMap = new
    for c in s:
        countMap.tryGetValue(c, out count)
        countMap[c] = count +1
    for c in s:
        if countMap[c] == 1:
            return c
    return null

char? (s):
    if s == null:
        throw
    unique = new
    repeating = new
    for c in s:
        if repeating.has(c):
            // do nothing
        else if unique.has(c):
            unique.remove(c)
            repeating.add(c)
        else:
            unique.add(c)
    for c in s:
        if unique.has(c):
            return c
    return null

char? (s):
    if s == null:
        throw
    dq = new            //{char}
    chToNodeMap = new   //{char, node{char}}
    repeating = new     //{char}
    for c in s:
        if repeating.has(c): // 3+ hit
            // do nothing
        else if chnodeMap.hasKey(c): // 2nd hit
            repeating.add(c)
            dq.deleteNode(chnodeMap[c])
            chnodeMap.removeKey(c)
        else: // 1st hit
            chnodeMap[c] = dq.enqueue(c)
    if dq.isEmpty():
        return null
    return dq.peek()

deque:
    node enqueue(T)
    bool deleteNode(node)
    T peek()
    bool isEmpty()

[Hat tip to MC]

// a        a
// abc      a
// aba      b
// abb      a
// abcbab   c
// aabb     null
char? (s):
    if s == null:
        throw
    consumed = new
    for i = s.length() -1, i >= 0, i--:
        c = s[i]
        if consumed.has(c):
            if firstUniqueChar == c:
                firstUniqueChar = null
        else:
            consumed.add(c)
            firstUniqueChar = c
    return firstUniqueChar