------
## 最终结论
```python
def maxLength(arr):
    def is_unique(s):
        return len(s) == len(set(s))
    
    def backtrack(index, current):
        nonlocal max_length
        if is_unique(current):
            max_length = max(max_length, len(current))
        for i in range(index, len(arr)):
            backtrack(i + 1, current + arr[i])
    
    max_length = 0
    backtrack(0, "")
    return max_length

# Example usage:
arr1 = ["un", "iq", "ue"]
arr2 = ["cha", "r", "act", "ers"]
arr3 = ["abcdefghijklmnopqrstuvwxyz"]

print(maxLength(arr1))  # Output: 4
print(maxLength(arr2))  # Output: 6
print(maxLength(arr3))  # Output: 26
```