'commit'

TangDou/AcWing/KMP/朴素版本求字符串的前缀函数.cpp

@@ -0,0 +1,64 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+/*
+aabaaab
+答案：
+[0,1,0,1,2,2,3]
+*/
+
+//朴素版本求字符串的前缀函数
+vector<int> prefix_function1(string s) {
+    int n = (int)s.length();
+    vector<int> pi(n);
+    for (int i = 1; i < n; i++)
+        for (int j = i; j >= 0; j--)
+            if (s.substr(0, j) == s.substr(i - j + 1, j)) {
+                pi[i] = j;
+                break;
+            }
+    return pi;
+}
+// 第一轮优化后的版本
+vector<int> prefix_function2(string s) {
+    int n = (int)s.length();
+    vector<int> pi(n);
+    for (int i = 1; i < n; i++)
+        for (int j = pi[i - 1] + 1; j >= 0; j--) // improved: j=i => j=pi[i-1]+1
+            if (s.substr(0, j) == s.substr(i - j + 1, j)) {
+                pi[i] = j;
+                break;
+            }
+    return pi;
+}
+// 第二轮优化后的版本
+vector<int> prefix_function3(string s) {
+    int n = (int)s.length();
+    vector<int> pi(n);
+    for (int i = 1; i < n; i++) {
+        int j = pi[i - 1];
+        while (j > 0 && s[i] != s[j]) j = pi[j - 1];
+        if (s[i] == s[j]) j++;
+        pi[i] = j;
+    }
+    return pi;
+}
+int main() {
+    string s = "aabaaab";
+    vector<int> res = prefix_function1(s);
+    for (int i = 0; i < (int)res.size(); i++)
+        cout << res[i] << " ";
+    puts("");
+
+    res = prefix_function2(s);
+    for (int i = 0; i < (int)res.size(); i++)
+        cout << res[i] << " ";
+    puts("");
+
+    res = prefix_function3(s);
+    for (int i = 0; i < (int)res.size(); i++)
+        cout << res[i] << " ";
+    puts("");
+
+    return 0;
+}