最长回文子串-python

dp方法

def longest_palindrome(s):
    # input: str   output: longest substing
    # 对于空字符、一个字符，都是其本身
    if not s:
        return ''
    n = len(s)
    print(f"字符长度:{n}")
    # state
    is_palindrome = [[False]*n for _ in range(n)]
    for i in range(n):
        is_palindrome[i][i] = True
    # 有无必要？
#     for i in range(1, n):
#         is_palindrome[i][i-1] = True
        
    longest, start, end = 1, 0, 0
    # 先循环区间长度，再循环区间起点
    for length in range(1, n):
        print(f"\n区间长度:{length}")
        for i in range(n-length):
            j = i + length     # 起点i，终点j
            is_palindrome[i][j] = s[i]==s[j] and is_palindrome[i+1][j-1]
            print(f"起点:{i}, 终点:{j}, s[i]:{s[i]}, s[j]:{s[j]}, {is_palindrome[i][j]}")                        
            if is_palindrome[i][j] and length+1 > longest:
                longest = length + 1
                start, end = i, j
    return s[start:end+1]
longest_palindrome("banana")