longest palindrome substring

Author: springuper

longestPalindrome.py

@@ -0,0 +1,70 @@
+# @return a string
+# DP approach, O(n^2)
+def longestPalindrome(s):
+    l = len(s)
+    maxStr = s[0]
+    maxLen = 1
+
+    p = [[0 for i in range(l)] for j in range(l)]
+    for i in range(l - 1):
+        p[i][i] = 1
+        if s[i] == s[i + 1]:
+            if maxLen < 2:
+                maxLen = 2
+                maxStr = s[i:(i + 2)]
+            p[i][i + 1] = 1
+        else:
+            p[i][i + 1] = 0
+
+    for k in range(2, l):
+        for i in range(0, l - k):
+            j = i + k
+            if p[i + 1][j - 1] and s[i] == s[j]:
+                p[i][j] = 1
+                if maxLen < (j - i + 1):
+                    maxLen = j - i + 1
+                    maxStr = s[i:(j + 1)]
+            else:
+                p[i][j] = 0
+
+    return maxStr
+
+# O(n)
+def longestPalindromeSubstring(s):
+    def preProcess(s):
+        if len(s) == 0:
+            return ''
+        ret = '^'
+        for c in s:
+            ret += '#' + c
+        return ret + '#$'
+
+    T = preProcess(s)
+    n = len(T)
+    C = 0
+    R = 0
+    P = [0 for i in range(n)]
+    for i in range(1, n - 1):
+        iMirror = C - (i - C)
+        P[i] = max(min(R - i, P[iMirror]), 0)
+
+        while T[i - P[i] - 1] == T[i + P[i] + 1]:
+            P[i] += 1
+
+        if P[i] + i > R:
+            R = P[i] + i
+            C = i
+
+    maxLen = 0
+    centerIndex = 0
+    for i in range(1, n - 1):
+        if P[i] > maxLen:
+            maxLen = P[i]
+            centerIndex = i
+
+    start = (centerIndex - 1 - maxLen) / 2
+    return s[start:(start + maxLen)];
+
+assert longestPalindromeSubstring('helloworld') == 'owo'
+assert longestPalindromeSubstring('abbc') == 'bb'
+assert longestPalindromeSubstring('spring') == 's'