Given a string input_digits consisting of digits from 2 to 9, return a list of all possible strings formed by mapping each digit to its corresponding set of letters, as defined by the standard telephone keypad mapping. The output list can be in any order. If input_digits is empty, return an empty list.
Example 1
Input: "34"
Output: [dg, dh, di, eg, eh, ei, fg, fh, fi]
Explanation: Digit 3 maps to [d,e,f], digit 4 maps to [g,h,i]. All combinations are listed.
Example 2
Input: "7"
Output: [p, q, r, s]
Explanation: Digit 7 maps to [p,q,r,s].
Example 3
Input: "29"
Output: [aw, ax, ay, az, bw, bx, by, bz, cw, cx, cy, cz]
Explanation: Digit 2 maps to [a,b,c], digit 9 maps to [w,x,y,z].
Constraints
Case 1
Input: "68"
Expected: ['mt', 'mu', 'mv', 'nt', 'nu', 'nv', 'ot', 'ou', 'ov']
Case 2
Input: "5"
Expected: ['j', 'k', 'l']
Case 3
Input: "82"
Expected: ['ta', 'tb', 'tc', 'ua', 'ub', 'uc', 'va', 'vb', 'vc']
Case 4
Input: "47"
Expected: ['gp', 'gq', 'gr', 'gs', 'hp', 'hq', 'hr', 'hs', 'ip', 'iq', 'ir', 'is']