【剑指Offer】58.1 翻转单词顺序列

Input:
"I am a student."

Output:
"student. a am I"

class Solution {
    public String reverseWords(String str) {
        str = str.trim();
        int r = str.length() - 1, l = r;
        StringBuilder res = new StringBuilder();
        
        // 从后往前
        while(l >= 0) {
            while(l >= 0 && str.charAt(l) != ' ') l--; // 找到空格
            res.append(str.substring(l+1, r+1) + " "); // 填补单词
            while(l >= 0 && str.charAt(l) == ' ') l--; // 跳过空格
            r = l;
        }
        return res.toString().trim();
    }
}

public String ReverseSentence(String str) {
    int n = str.length();
    char[] chars = str.toCharArray();
    int i = 0, j = 0;
    while (j <= n) {
        if (j == n || chars[j] == ' ') {
            reverse(chars, i, j - 1);
            i = j + 1;
        }
        j++;
    }
    reverse(chars, 0, n - 1);
    return new String(chars);
}

private void reverse(char[] c, int i, int j) {
    while (i < j)
        swap(c, i++, j--);
}

private void swap(char[] c, int i, int j) {
    char t = c[i];
    c[i] = c[j];
    c[j] = t;
}