kmp字符串匹配算法

algorithm/kmp.cpp

@@ -0,0 +1,55 @@
+/**
+ * KMP字符串匹配算法
+ * https://www.zhihu.com/question/21923021/answer/1032665486
+ */
+#include <iostream>
+#include <vector>
+#include <string_view>
+
+using namespace std;
+
+// 生成KMP pattern的next数组,是一个DP算法
+vector<int> make_next(string_view str) {
+    int n = str.size();
+    // 字符串str[0...i]的最大前后缀为next[i]
+    vector<int> next(n, 0);
+    for (int i = 1; i < n; ++i) {
+        int k = next[i - 1];
+        while (true) {
+            // if next[i - 1] == k and str[k] = str[i] then next[i] >= k + 1
+            // if next[i] = k + x + 1 then next[i - 1] >= k + x
+            if (str[k] == str[i]) {
+                next[i] = k + 1;
+                break;
+            } else {
+                if (k != 0) k = next[k - 1];
+                else break;
+            }
+        }
+    }
+    return next;
+}
+
+
+// KMP字符串匹配算法:输出pattern在text第一次出现的位置
+int KMP(string_view text, string_view pattern) {
+    int n = text.size();
+    int m = pattern.size();
+    if (m == 0) return 0;
+    if (n < m) return -1;
+    vector<int> next = make_next(pattern);
+    // text的下标始终不回退
+    for (int i = 0, j = 0; i < n; ++i) {
+        // 当遇到不匹配时,按next移动,避免不必要的尝试
+        while (j != 0 && text[i] != pattern[j]) j = next[j - 1];
+        if (text[i] == pattern[j]) ++j;
+        if (j == m) return i - m + 1;
+    }
+    return -1;
+}
+
+int main() {
+    cout << KMP("hello", "ll") << endl;
+    cout << KMP("test_ts", "tet") << endl;
+    return 0;
+}