Python：簡單的 If-Else 語句不起作用

Question

我正在編寫一個動態編程 function，它找到了 Python 中最長的 substring 回文：

def longestPalindrome(s: str) -> str:
        dict = {}
        start = 0
        end = 0

        # initialize base cases and left of base cases
        for i in range(len(s)):
            dict[(i,i)] = 1

        for y in range(len(s), -1, -1):
            for x in range(y, len(s)):
                print((x,y))
                # memo check
                if (x, y) not in dict.keys():
                    if s[x] == s[y]:
                        if x - y == 1:
                            dict[(x,y)] = 1
                            if (end - start) <= (x-y):
                                start = y
                                end = x

                        elif dict[(x-1,y+1)] == 1: # if substr betweet x & y is palindrome
                            dict[(x,y)] = 1
                            if (end - start) <= (x-y):
                                start = y
                                end = x                        
                    else:      # ISSUE HERE
                        dict[(x,y)] = 0

        return s[start:end+1]

在 function 中輸入“aaabaa”時出現以下錯誤：KeyError : (4, 1)

當我進行以下更改時，它會正確輸出

def longestPalindrome(s: str) -> str:
        dict = {}
        start = 0
        end = 0

        # initialize base cases and left of base cases
        for i in range(len(s)):
            dict[(i,i)] = 1

        for y in range(len(s), -1, -1):
            for x in range(y, len(s)):
                print((x,y))
                # memo check
                if (x, y) not in dict.keys():
                    dict[(x,y)] = 0  # **CHANGE**
                    if s[x] == s[y]:
                        if x - y == 1:
                            dict[(x,y)] = 1
                            if (end - start) <= (x-y):
                                start = y
                                end = x

                        elif dict[(x-1,y+1)] == 1: # if substr betweet x & y is palindrome
                            dict[(x,y)] = 1
                            if (end - start) <= (x-y):
                                start = y
                                end = x                        
#                     else:         # **REMOVE**
#                         dict[(x,y)] = 0

        return s[start:end+1]

我很確定這兩個版本的函數在邏輯上是相同的，但我無法弄清楚為什么第一個版本不起作用。

Answer 1

您是否打算將else one tab 寬度放回去？ 你的else應該與記憶步驟相匹配。 它與if s[x] == s[y]: