feat: 最长回文子串-中心扩散法

Author: sl1673495

动态规划/最长回文子串-5.js

@@ -1,4 +1,47 @@
 /**
+ * 动态规划
+ * @param {string} s
+ * @return {string}
+ */
+// let longestPalindrome = function (s) {
+//   let n = s.length
+//   if (n < 2) {
+//     return s
+//   }
+
+//   let dp = []
+//   for (let i = 0; i < n; i++) {
+//     dp[i] = []
+//     dp[i][i] = true
+//   }
+
+//   let max = 0
+//   let begin = 0
+//   for (let j = 1; j < n; j++) {
+//     for (let i = 0; i < j; i++) {
+//       if (s[j] !== s[i]) {
+//         dp[i][j] = false
+//       } else {
+//         let indent = dp[i + 1][j - 1]
+//         if (indent === undefined || indent === true) {
+//           dp[i][j] = true
+//         }else {
+//           dp[i][j] = false
+//         }
+//       }
+
+//       if (dp[i][j] === true && j - i > max) {
+//         max = j - i
+//         begin = i
+//       }
+//     }
+//   }
+//   console.log('dp', dp)
+//   return s.substr(begin, max + 1)
+// }
+
+/**
+ * 中心扩散法
  * @param {string} s
  * @return {string}
  */
@@ -8,35 +51,27 @@ let longestPalindrome = function (s) {
     return s
   }
 
-  let dp = []
-  for (let i = 0; i < n; i++) {
-    dp[i] = []
-    dp[i][i] = true
-  }
-
-  let max = 0
   let begin = 0
-  for (let j = 1; j < n; j++) {
-    for (let i = 0; i < j; i++) {
-      if (s[j] !== s[i]) {
-        dp[i][j] = false
-      } else {
-        let indent = dp[i + 1][j - 1]
-        if (indent === undefined || indent === true) {
-          dp[i][j] = true
-        }else {
-          dp[i][j] = false
-        }
-      }
+  let max = 1
 
-      if (dp[i][j] === true && j - i > max) {
-        max = j - i
-        begin = i
+  let spread = (start, end) => {
+    while (s[start] === s[end] && start >= 0 && end < n) {
+      let len = end - start + 1
+      if (len > max) {
+        max = len
+        begin = start
       }
+      start--
+      end++
     }
   }
-  console.log('dp', dp)
-  return s.substr(begin, max + 1)
+
+  for (let mid = 0; mid < n; mid++) {
+    spread(mid, mid)
+    spread(mid, mid + 1)
+  }
+
+  return s.substr(begin, max)
 }
 
-console.log(longestPalindrome("abcda"))
+console.log(longestPalindrome("babad"))

回文子串-647.js

@@ -0,0 +1,29 @@
+/**
+ * @param {string} s
+ * @return {number}
+ */
+let countSubstrings = function (s) {
+  let n = s.length
+  if (n < 2) {
+    return n
+  }
+
+  let count = 0
+
+  let spread = (start, end) => {
+    while (s[start] === s[end] && start >= 0 && end < n) {
+      start--
+      end++
+      count++
+    }
+  }
+
+  for (let mid = 0; mid < n; mid++) {
+    spread(mid, mid)
+    spread(mid, mid + 1)
+  }
+
+  return count
+}
+
+console.log(countSubstrings("a"))