var TEST = function () {
    return {
        exec: function (str, rowNum) {
            let rtn = '', gap = rowNum * 2 - 2;
            if (gap <= 0) { return str; }
            
            const usedIdx = new Map();
            for (let i=0; i<rowNum; ++i) {
                if (!str[i]) { break; }
                rtn += str[i];
                usedIdx.set(i, true);
                
                let range = 1, inv = -1;
                while(true) {
                    let ptr = range * gap + i * inv
                    if (!usedIdx.has(ptr)) {
                        if (!str[ptr]) { break; }
                        rtn += str[ptr];
                        usedIdx.set(ptr, true);
                    }
                    inv *= -1;
                    if (inv === -1) { range++; }
                }
            }
            
            return rtn;
        }
    };
}();