为什么这段代码没有产生正确的输出？ [英] Why doesn't this code produce the correct output?

问题描述

给定一个字符串和一个数字，此函数将生成该字符串中所有可能的n个字母的组合。

Given a string and a number, this function will produce all possible combinations of n letters from that string.

如果我传递abcd和3，那么我应该得到以下输出：

i.e. If I pass "abcd" and 3, then I should get the following output:

abc
abd
acd
bcd

这是代码

- (void)viewDidLoad {
    [super viewDidLoad];
    [self expand_combinations:@"abcd" arg2:@"" arg3:3];
}

-(void) expand_combinations: (NSString *) remaining_string arg2:(NSString *)s arg3:(int) remain_depth
{
    if(remain_depth==0)
    {
        printf("%s\n",[s UTF8String]);
        return;
    }

    for(int k=0; k < [remaining_string length]; ++k)
    {
        s = [s stringByAppendingString:[[remaining_string substringFromIndex:k] substringToIndex:1]];
        [self expand_combinations:[remaining_string substringFromIndex:k+1] arg2:s arg3:remain_depth - 1];
    }
    return;
}

而是这是打印输出

abc
abcd
abcd
abcd

推荐答案

只是一个快速而脏的解决方案（没有重命名变量名等）

Just a quick and dirty solution (didn't rename variable names and so on) for your function

-(void) expand_combinations: (NSString *) remaining_string arg2:(NSString *)s arg3:(int) remain_depth
{
    NSString *newString = [remaining_string stringByReplacingCharactersInRange:NSMakeRange(remain_depth, 1) withString:@""];
    // NSLog(newString);

    if(remain_depth==0)
    {
        printf("%s\n",[s UTF8String]);
        return;
    }
    [self expand_combinations:remaining_string arg2:s arg3:remain_depth - 1];
    return;
}

输出（我使用NSLog）

Output (I used NSLog)

2010-09-26 04:27:26.462 Untitled[5417:207] abc
2010-09-26 04:27:26.462 Untitled[5417:207] abd
2010-09-26 04:27:26.463 Untitled[5417:207] acd
2010-09-26 04:27:26.467 Untitled[5417:207] bcd

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