分析几个写Python代码犯的错误

原题是lc320

Java代码：

public class Solution {
    public List generateAbbreviations(String word){
        List ans = new ArrayList();
        backtrack(ans, new StringBuilder(), word, 0, 0);
        return ans;
    }

    // i is the current position
    // k is the count of consecutive abbreviated characters
    private void backtrack(List ans, StringBuilder builder, String word, int i, int k){
        int len = builder.length(); // keep the length of builder
        if(i == word.length()){
            if (k != 0) builder.append(k); // append the last k if non zero
            ans.add(builder.toString());
        } else {
            // the branch that word.charAt(i) is abbreviated
            backtrack(ans, builder, word, i + 1, k + 1);

            // the branch that word.charAt(i) is kept
            if (k != 0) builder.append(k);
            builder.append(word.charAt(i));
            backtrack(ans, builder, word, i + 1, 0);
        }
        builder.setLength(len); // reset builder to the original state
    }
}

错误1：

用Python的string代替string builder。因为string是immutable，在下一次递归时不会改变。

错误2：

class Solution(object):
    def generateAbbreviations(self, word):
        """
        :type word: str
        :rtype: List[str]
        """
        def backtrack(ans, builder, word, i, k):
            length = len(builder)
            if i == len(word):
                if k != 0:
                    builder.append(str(k))
                ans.append("".join(builder[:]))
                # return  # don't do this, once you return, some branch can't call pop
            else:
                backtrack(ans, builder, word, i+1, k+1)
                if k != 0:
                    builder.append(str(k))
                builder.append(word[i])
                backtrack(ans, builder, word, i+1, 0)
            # builder = builder[:length]  # dont do this, slicing produces a new copy, this builder is that builder before!!!
            while len(builder) != length:
                builder.pop()
        
        ans = []
        backtrack(ans, [], word, 0, 0)
        return ans
        

用builder = builder[:length]来代替Java的setLength，错误在于Python切片会产生新的list，此builder非builder

错误3：多写一个return（如上所示）。

这会导致有些分支在返回以后没办法执行pop()。