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docs/notes/38. 字符串的排列.md

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+# 38. 字符串的排列
+
+[NowCoder](https://www.nowcoder.com/practice/fe6b651b66ae47d7acce78ffdd9a96c7?tpId=13&tqId=11180&tPage=1&rp=1&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking&from=cyc_github)
+
+## 题目描述
+
+输入一个字符串，按字典序打印出该字符串中字符的所有排列。例如输入字符串 abc，则打印出由字符 a, b, c 所能排列出来的所有字符串 abc, acb, bac, bca, cab 和 cba。
+
+## 解题思路
+
+```java
+private ArrayList<String> ret = new ArrayList<>();
+
+public ArrayList<String> Permutation(String str) {
+    if (str.length() == 0)
+        return ret;
+    char[] chars = str.toCharArray();
+    Arrays.sort(chars);
+    backtracking(chars, new boolean[chars.length], new StringBuilder());
+    return ret;
+}
+
+private void backtracking(char[] chars, boolean[] hasUsed, StringBuilder s) {
+    if (s.length() == chars.length) {
+        ret.add(s.toString());
+        return;
+    }
+    for (int i = 0; i < chars.length; i++) {
+        if (hasUsed[i])
+            continue;
+        if (i != 0 && chars[i] == chars[i - 1] && !hasUsed[i - 1]) /* 保证不重复 */
+            continue;
+        hasUsed[i] = true;
+        s.append(chars[i]);
+        backtracking(chars, hasUsed, s);
+        s.deleteCharAt(s.length() - 1);
+        hasUsed[i] = false;
+    }
+}
+```