JavaScript simple solution (~15 lines)
Beat 75%, not the most optimal but simple enough.\n\n\n/**\n * @param {string[]} arr\n * @return {number}\n */\nvar maxLength = function (arr) {\n    let res = 
Beat 75%, not the most optimal but simple enough.\n\n```\n/**\n * @param {string[]} arr\n * @return {number}\n */\nvar maxLength = function (arr) {\n    let res = 0;\n\n    let solve = (start, curr) => {\n        if (curr.length !== new Set(curr.split(\'\')).size) {\n            return;\n        }\n        \n        res = Math.max(res, curr.length);\n\n        for (let i = start; i < arr.length; i++) {\n            solve(i + 1, `${curr}${arr[i]}`);\n        }\n    }\n\n    solve(0, \'\');\n\n    return res;\n};\n```
['JavaScript']