Update 28-implement-strStr().js

Author: everthis

28-implement-strStr().js

@@ -1,3 +1,35 @@
+/**
+ * @param {string} haystack
+ * @param {string} needle
+ * @return {number}
+ */
+const strStr = function(a, b) {
+  if(b === '') return 0
+  if(a.length < b.length) return -1
+  if(a.length === b.length) return a === b ? 0 : -1
+  const m = a.length, n = b.length
+  const fail = Array(n).fill(-1)
+  // DFA
+  for(let i = 1; i < n; i++) {
+    let j = fail[i - 1]
+    while(j !== -1 && b[j + 1] !== b[i]) {
+      j = fail[j]      
+    }
+    if(b[j + 1] === b[i]) fail[i] = j + 1
+  }
+  let pos = -1
+  for(let i = 0; i < m; i++) {
+    while(pos !== -1 && a[i] !== b[pos + 1]) pos = fail[pos]
+    if(a[i] === b[pos + 1]) {
+      pos++
+      if(pos === n - 1) return i - n + 1
+    }
+  }
+  return -1
+};
+
+// another
+
 /**
  * @param {string} haystack
  * @param {string} needle