leetcode5 最长回文子串

'''
递推式:
1 一般: S[i+1] == S[j-1] and j>i and j-i == len(S[i+1,j-1])
2 初始化dp矩阵对角线的值为 true,相邻两个元素相等时dp[i][i+1] 为true ,初始化回文串起始位置和长度。
'''
def longestPalindrome(s):
    n = len(s)
    if s == '':
        return ''
    if n == 1:
        return s
    start = 0
    max_len = 0
    dp = [[False for i in range(n)] for j in range(n)]
    for i in range(n):
        dp[i][i] = True
        if i + 1 < n:
            #####如果两个连续字符相等,则i,i+1 为true
            if s[i] == s[i + 1]:
                dp[i][i + 1] = True
                start = i
                max_len = 2
        if max_len == 0:
            max_len = 1
            start = 0
    '''
    前面的已经遍历到回文串长度为2,从长度为3开始,|i-j|=2开始
    strlen [3,n+1] 
    '''
    ###dp[i][j]表示i到j是否是回文串,i < j 能保证不越界
    for strlen in range(3, n + 1):
        for i in range(n - strlen + 1):
            j = i + strlen - 1
            if s[i] == s[j] and dp[i + 1][j - 1]:
                dp[i][j] = True
                max_len = strlen
                start = i
    return s[start:start + max_len]

马拉车算法

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