leetcode 日经贴，Cpp code -Repeated DNA Sequences

class Solution {
public:
int getnum(char ch) {
int ret = 0;
switch(ch) {
case 'A': ret = 0; break;
case 'C': ret = 1; break;
case 'G': ret = 2; break;
case 'T': ret = 3; break;
default: ret = 0;
}
return ret;
}
vector<string> findRepeatedDnaSequences(string s) {
vector<string> ans;
int n = s.length();
if (n <= 10) {
return ans;
}
int base = 4, m = 0, highbase = (1 << 18);
for (int i = 0; i < 10; ++i) {
m = m * base + getnum(s[i]);
}
unordered_map<int, int> mp;
mp[m] = 1;
for (int i = 10; i < n; ++i) {
m -= highbase * getnum(s[i - 10]);
m = m * base + getnum(s[i]);
if (mp.find(m) != mp.end()) {
if (mp[m] == 1) {
mp[m] = -1;
ans.push_back(s.substr(i - 9, 10));
}
} else {
mp[m] = 1;
}
}
return ans;
}
};