Given an equation, represented by words on left side and the result on right side.
You need to check if the equation is solvable under the following rules:

Each character is decoded as one digit (0 - 9).
Every pair of different characters they must map to different digits.
Each words[i] and result are decoded as one number without leading zeros.
Sum of numbers on left side (words) will equal to the number on right side (result). 

Return True if the equation is solvable otherwise return False.
 
Example 1:
Input: words = ["SEND","MORE"], result = "MONEY"
Output: true
Explanation: Map 'S'-> 9, 'E'->5, 'N'->6, 'D'->7, 'M'->1, 'O'->0, 'R'->8, 'Y'->'2'
Such that: "SEND" + "MORE" = "MONEY" ,  9567 + 1085 = 10652
Example 2:
Input: words = ["SIX","SEVEN","SEVEN"], result = "TWENTY"
Output: true
Explanation: Map 'S'-> 6, 'I'->5, 'X'->0, 'E'->8, 'V'->7, 'N'->2, 'T'->1, 'W'->'3', 'Y'->4
Such that: "SIX" + "SEVEN" + "SEVEN" = "TWENTY" ,  650 + 68782 + 68782 = 138214
Example 3:
Input: words = ["THIS","IS","TOO"], result = "FUNNY"
Output: true

Example 4:
Input: words = ["LEET","CODE"], result = "POINT"
Output: false

 
Constraints:

2 <= words.length <= 5
1 <= words[i].length, result.length <= 7
words[i], result contains only upper case English letters.
Number of different characters used on the expression is at most 10.

```python

USER_FILENAME = "<user_code>"
import sys, json, builtins
import math, heapq, bisect, itertools, functools, collections, statistics, fractions, decimal, operator, array, random, re, string, os, pathlib
try:
    import resource as _res
except Exception:
    _res = None
from collections import Counter, defaultdict, deque, OrderedDict
from functools import lru_cache, reduce
from itertools import accumulate, permutations, combinations, product
from math import gcd, ceil, floor, sqrt
from decimal import Decimal
try:
    from typing import *
except Exception:
    pass
try:
    sys.setrecursionlimit(1_000_000)
except Exception:
    pass
try:
    import site as _site
    try:
        _sys_sites = _site.getsitepackages() if hasattr(_site, "getsitepackages") else []
        for _p in _sys_sites:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
    try:
        _usp = _site.getusersitepackages()
        if isinstance(_usp, str):
            _usp = [_usp]
        for _p in _usp:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
except Exception:
    pass

def _disabled_input(*args, **kwargs):
    raise RuntimeError("input() is disabled in callable mode")
builtins.input = _disabled_input

ns = {"__name__": "__judge__"}  # prevent if __name__ == "__main__" blocks
for _k, _v in list(globals().items()):
    if not _k.startswith("_"):
        ns[_k] = _v
_CODE = 'class Solution:\n    def isSolvable(self, words: List[str], result: str) -> bool:\n        self.words = words\n        self.result = result\n        self.front_chars = set([word[0] for word in words] + [result[0]])    \n        self.result_indices = []\n        self.all_chars = []\n        \n        new_words = words + [result]\n        while len(new_words) > 0:\n            for i in range(len(new_words)):\n                self.all_chars.append(new_words[i][-1])\n                new_words[i] = new_words[i][:-1]\n            self.result_indices.append(len(self.all_chars) - 1)\n            new_words = [word for word in new_words if len(word) > 0]\n\n        self.mappings = {}\n        self.used_chars = set()\n        \n        return self.backtrack(0, 0)\n        \n    def backtrack(self, current_i: int, carry: int) -> bool:\n        if current_i == len(self.all_chars):\n            return True\n        \n        cur_char = self.all_chars[current_i]\n\n        if current_i in self.result_indices:\n            code, new_carry = self.verify(self.result_indices.index(current_i), carry)\n            if code == 0:\n                return False\n            else:\n                if self.backtrack(current_i + 1, new_carry):\n                    return True\n                \n                if code == 2:\n                    self.used_chars.remove(self.mappings[cur_char])\n                    del self.mappings[cur_char]\n                    \n                return False\n                    \n        if cur_char in self.mappings:\n            return self.backtrack(current_i + 1, carry)\n        \n        for i in range(10):\n            if cur_char in self.front_chars and i == 0:\n                continue\n            if i not in self.used_chars:\n                self.mappings[cur_char] = i\n                self.used_chars.add(i)\n                \n                if self.backtrack(current_i + 1, carry):\n                    return True\n\n                del self.mappings[cur_char]\n                self.used_chars.remove(i)\n                \n        return False\n            \n    def verify(self, index: int, carry: int) -> (int, int):\n        cur_sum = carry\n            \n        for word in self.words:\n            if index < len(word):\n                cur_sum += self.mappings[word[index * -1 -1]]\n\n        carry = int(cur_sum / 10)\n        cur_sum = cur_sum % 10\n            \n        result_char = self.result[index * -1 - 1]\n        if result_char in self.mappings:\n            if self.mappings[result_char] != cur_sum:\n                return 0, 0\n            else:\n                return 1, carry\n        else:\n            if cur_sum in self.used_chars:\n                return 0, 0\n            if result_char in self.front_chars and cur_sum == 0:\n                return 0, 0\n            \n            self.mappings[result_char] = cur_sum\n            self.used_chars.add(cur_sum)\n            return 2, carry\n        \n\n                \n'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'isSolvable'
if _fn_name:
    if "Solution" in ns:
        try:
            _obj = ns["Solution"]()
            if hasattr(_obj, _fn_name):
                _target = getattr(_obj, _fn_name)
        except Exception:
            _target = None
    if _target is None and _fn_name in ns and callable(ns[_fn_name]):
        _target = ns[_fn_name]
# Fallback: if there's exactly one user-defined function, use it
if _target is None:
    _cands = [v for k, v in ns.items() if callable(v) and getattr(v, "__module__", "") in ("__judge__", "__main__")]
    _user_funcs = [c for c in _cands if getattr(c, "__name__", "").startswith("_") is False]
    if len(_user_funcs) == 1:
        _target = _user_funcs[0]
if _target is None:
    raise RuntimeError(f"Could not resolve callable '{_fn_name}'")

_raw = sys.stdin.read().strip()
if _raw:
    _parsed = json.loads(_raw)
    if isinstance(_parsed, dict):
        _kwargs = _parsed.get("kwargs", {})
        _args = _parsed.get("args", [])
    else:
        _args, _kwargs = _parsed, {}
else:
    _args, _kwargs = [], {}
if not isinstance(_args, (list, tuple)):
    _args = [_args]
_res = _target(*_args, **_kwargs)
try:
    sys.stdout.write(repr(_res) + "\n")
except Exception:
    pass

```

Measured memory_bytes: 8804