制造偶串的O(n)算法，应用KMP的next数组。 #include <bits/stdc++.h> using namespace std; int solve(string& s) { vector<int> next(s.size(), 0); int j = 0, i = 1; while (i < s.size()) { if (s[i] == s[j]) { next[i] = j+1; ++i; ++j; } else if (j != 0) { j = next[j-1]; } else { ++i; } } for (i = next.size() - 3; i >= 0; i -= 2) { if (next[i] * 2 == i + 1 || next[i] == i) { return i + 1; } } return 0; } int main() { freopen("in.txt", "r", stdin); string s; while (cin >> s) { cout << solve(s) << endl; } return 0; }