bool canCross(vector<int>& stones) {

if (stones[1] != 1) {

return false;

}

unordered_map<int, unordered_set<int>> last_jump_units;

for (const auto& s: stones) {

last_jump_units.emplace(s, {unordered_set<int>()});

}

last_jump_units[1].emplace(1);

for (int i = 0; i + 1 < stones.size(); ++i) {

for (const auto& j : last_jump_units[stones[i]]) {

for (const auto& k : {j - 1, j, j + 1}) {

if (k > 0 && last_jump_units.count(stones[i] + k)) {

last_jump_units[stones[i] + k].emplace(k);

}

}

}

}

return !last_jump_units[stones.back()].empty();

}