LeetCode 344 Reverse String

原题如下：

Write a function that takes a string as input and returns the string reversed.

Example:
Given s = "hello", return "olleh".

简单字符串翻转，直接上代码。

    public String reverseString(String s) {
        StringBuilder sb = new StringBuilder();
        for (int i = s.length() - 1; i >= 0; i--) {
            sb.append(s.charAt(i));
        }
        return sb.toString();
    }

如何实现原地翻转呢？由于String不能修改，所以需要修改一下方法定义，参数使用StringBuilder。

    public void reverseStringInPlace(StringBuilder sb) {
        reverseStringInPlace(sb, 0, sb.length() - 1);
    }

    public void reverseStringInPlace(StringBuilder sb, int start, int end) {
        if (start >= end || end > sb.length() - 1) {
            return;
        }
        for (int i = start, j = end; i < j; i++, j--) {
            char tmp = sb.charAt(i);
            sb.setCharAt(i, sb.charAt(j));
            sb.setCharAt(j, tmp);
        }
    }

这里又引出另一个问题，如何将字符串中的单词顺序翻转，但是单词本身不变？

举例:
Given s = "hello world", return "world hello".

解决方法：先对每个单词做翻转，然后对整个字符串做翻转即可。

    public void reverseSentenceInPlace(StringBuilder sb) {
        int i = 0, j = 0;
        for (; j < sb.length(); j++) {
            if (!Character.isLetter(sb.charAt(j))) {
                if (i < j) {
                    reverseStringInPlace(sb, i, j - 1);
                }
                i = j;
                i++;
            }
        }
        if (i < j) {
            reverseStringInPlace(sb, i, j - 1);
        }
        reverseStringInPlace(sb, 0, sb.length() - 1);
    }

参考源码：https://github.com/pkufork/Martians/blob/master/src/main/java/com/pkufork/martians/leetcode/L344_ReverseString.java