5. 最长回文子串
https://leetcode-cn.com/problems/longest-palindromic-substring/
解法一:暴力
两个for循环,外层i遍历每个字符,然后内层j从i分别向两侧出发,找两头是否相等,不相等就退出循环。每次找到一个回文串,求长度,并记录首尾位置。
class Solution {
public:
string longestPalindrome(string s) {
int maxLen = 0;
int n = s.length();
int start = 0, end = 0;
for (int i = 0; i < n; i++) {
for (int j = 0; i-j >= 0 && i+j < n && s[i+j] == s[i-j]; j++) { //回文长度为奇
if (2*j+1 > maxLen) { // len = (i+j)-(i-j)+1 = 2*j+1
start = i-j;
end = i+j;
maxLen = 2*j+1;
}
}
for (int j = 0; i-1-j >= 0 && i+j < n && s[i-1-j] == s[i+j]; j++) { //回文长度为偶
if(2*j+2 > maxLen) { // len = (i+j)-(i-1-j)+1 = 2*j+2
start = i-1-j;
end = i+j;
maxLen = 2*j+2;
}
}
}
return s.substr(start, end-start+1);
}
};
#python3:注意将c++中带复杂条件的for语句转换为py时,
#只能转成while语句来判断条件,在循环外设j初始值,在语句最后j自增
class Solution:
def longestPalindrome(self, s: str) -> str:
maxLen = 0
n = len(s)
start = 0
end = 0
for i in range(n):
j = 0
while i-j >= 0 and i+j < n and s[i+j] == s[i-j]:
if 2*j+1 > maxLen:
start = i-j
end = i+j
maxLen = 2*j+1
j += 1
j = 0
while i-1-j >= 0 and i+j < n and s[i-1-j] == s[i+j]:
if 2*j+2 > maxLen:
start = i-1-j
end = i+j
maxLen = 2*j+2
j += 1
return s[start:end+1]
解法二:DP
dp(i, j)
represents whether s(i ... j)
can form a palindromic substring, dp(i, j)
is true when s(i) equals to s(j) and s(i+1 ... j-1)
is a palindromic substring. When we found a palindrome, check if it’s the longest one. Time complexity O(n^2). 需要注意的是构造初始解时,i是从n-1开始,为何?
看下图可知,对于串“aaaa”,由于i<=j
,因此矩阵只有上三角。初始化斜对角全为1(因为单个字符必为回文串),第二条对角线根据是否有i==j
决定(两个字符只有a==b才为回文)。其余每个元素(i, j)
根据其左下方元素(i+1, j-1)
决定。

class Solution:
def longestPalindrome(self, s: str) -> str:
maxLen = 0
n = len(s)
res = ""
dp = [[False] * n for _ in range(n)] #二维数组创建
for i in range(n-1, -1, -1):
for j in range(i, n):
if i == j: #对角线
dp[i][j] = True
elif j-i == 1: #第二条对角线
dp[i][j] = s[i] == s[j]
else: #其余位置
dp[i][j] = dp[i+1][j-1] and s[i] == s[j]
#找到更长的回文串
if dp[i][j] and j-i+1 > maxLen:
maxLen = j-i+1
res = s[i:j+1]
return res
实际上j由大到小也可以,因为填充方式是从下往上,因此只要保证i从大到小即可,j顺序可以随意,下一层都已经填完
class Solution:
def longestPalindrome(self, s: str) -> str:
maxLen = 0
n = len(s)
res = ""
dp = [[False] * n for _ in range(n)] #二维数组创建
for i in range(n-1, -1, -1):
for j in range(n-1, i-1, -1):
if i == j: #对角线
dp[i][j] = True
elif j-i == 1: #第二条对角线
dp[i][j] = s[i] == s[j]
else: #其余位置
dp[i][j] = dp[i+1][j-1] and s[i] == s[j]
#找到更长的回文串
if dp[i][j] and j-i+1 > maxLen:
maxLen = j-i+1
res = s[i:j+1]
return res
然而运行时dp比暴力慢许多,不知为何
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