• https://leetcode-cn.com/problems/to-lower-case/
• https://leetcode-cn.com/problems/length-of-last-word/
• https://leetcode-cn.com/problems/jewels-and-stones/
• https://leetcode-cn.com/problems/first-unique-character-in-a-string/
• https://leetcode-cn.com/problems/string-to-integer-atoi/

1 转换成小写字母

709. 转换成小写字母

    public String toLowerCase(String str) {
        String ans = "";
        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            if (c >= 'A' && c <= 'Z') {
                c = (char) (c + 32);
            }
            ans += c;
        }
        return ans;
    }

    public String toLowerCase2(String str) {
        char[] a = str.toCharArray();
        for (int i = 0; i < a.length; i++)
            if ('A' <= a[i] && a[i] <= 'Z')
                a[i] = (char) (a[i] - 'A' + 'a');//不需要知道a和A之间相差32
        return new String(a);
    }

2 最后一个单词的长度

58. 最后一个单词的长度

3 宝石与石头

771. 宝石与石头

字符串中的第一个唯一字符

387. 字符串中的第一个唯一字符

    public int firstUniqChar(String s) {
        int[] count = new int[26];// 存储各字符出现次数
        int n = s.length();
        for (int i = 0; i < n; i++) {
            count[s.charAt(i) - 'a']++;
        }
        for (int i = 0; i < n; i++) {
            if (count[s.charAt(i) - 'a'] == 1) {
                return i;
            }
        }
        return -1;// 无解
    }

      /**
        * 如果题目要求不仅仅是小写字母，而是任意字符，可以使用HashMap
        */
    public int firstUniqChar(String s) {
        HashMap<Character, Integer> count = new HashMap<Character, Integer>();
        int n = s.length();
        for (int i = 0; i < n; i++) {
            char c = s.charAt(i);
            count.put(c, count.getOrDefault(c, 0) + 1);
        }
        for (int i = 0; i < n; i++) {
            if (count.get(s.charAt(i)) == 1) {
                return i;
            }
        }
        return -1;
    }