leetcode-1370. 上升下降字符串

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class Solution {
    public String sortString(String s) {
        char[] ch = s.toCharArray();
        Arrays.sort(ch);
        int n = ch.length;
        // 定义一个访问过的数组
        boolean[] st = new boolean[n];
        StringBuilder sb = new StringBuilder();

        while (sb.length() != n) {
            // 记录前一个字符
            char pre = 'A';
            for (int k = 0; k < n; k++) {
                if (!st[k] && ch[k] != pre){
                    sb.append(ch[k]);
                    st[k] = true;
                    pre = ch[k];
                }
            }
            pre = 'A';
            for (int k = n-1; k >= 0; k--) {
                if (!st[k] && ch[k] != pre){
                    sb.append(ch[k]);
                    st[k] = true;
                    pre = ch[k];
                }
            }
        }
        return sb.toString();
    }
}