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1.什么暴力递归可以继续优化？

有重复调用同一个子问题的解，这种递归可以优化

如果每一个子问题都是不同的解，无法优化也不用优化

2.暴力递归和动态规划的关系

某一个暴力递归，有解的重复调用，就可以把这个暴力递归优化成动态规划

任何动态规划问题，都一定对应着某一个有重复过程的暴力递归

但不是所有的暴力递归，都一定对应着动态规划

3.如何找到某个问题的动态规划方式？

1）设计暴力递归：重要原则+4种常见尝试模型！重点！

2）分析有没有重复解：套路解决

3）用记忆化搜索 -> 用严格表结构实现动态规划：套路解决

4）看看能否继续优化：套路解决

4.面试中设计暴力递归过程的原则

1）每一个可变参数的类型，一定不要比int类型更加复杂

2）原则1）可以违反，让类型突破到一维线性结构，那必须是单一可变参数

3）如果发现原则1）被违反，但不违反原则2），只需要做到记忆化搜索即可

4）可变参数的个数，能少则少

5.常见的4种尝试模型

1）从左往右的尝试模型

2）范围上的尝试模型

3）多样本位置全对应的尝试模型

4）寻找业务限制的尝试模型

习题1 打印n层汉诺塔从最左边移动到最右边的全部过程

public static void hanoi1(int n) {leftToRight(n);}// 请把1~N层圆盘 从左 -> 右public static void leftToRight(int n) {if (n == 1) { // base caseSystem.out.println("Move 1 from left to right");return;}leftToMid(n - 1);System.out.println("Move " + n + " from left to right");midToRight(n - 1);}// 请把1~N层圆盘 从左 -> 中public static void leftToMid(int n) {if (n == 1) {System.out.println("Move 1 from left to mid");return;}leftToRight(n - 1);System.out.println("Move " + n + " from left to mid");rightToMid(n - 1);}public static void rightToMid(int n) {if (n == 1) {System.out.println("Move 1 from right to mid");return;}rightToLeft(n - 1);System.out.println("Move " + n + " from right to mid");leftToMid(n - 1);}public static void midToRight(int n) {if (n == 1) {System.out.println("Move 1 from mid to right");return;}midToLeft(n - 1);System.out.println("Move " + n + " from mid to right");leftToRight(n - 1);}public static void midToLeft(int n) {if (n == 1) {System.out.println("Move 1 from mid to left");return;}midToRight(n - 1);System.out.println("Move " + n + " from mid to left");rightToLeft(n - 1);}public static void rightToLeft(int n) {if (n == 1) {System.out.println("Move 1 from right to left");return;}rightToMid(n - 1);System.out.println("Move " + n + " from right to left");midToLeft(n - 1);}public static void hanoi2(int n) {if (n > 0) {func(n, "left", "right", "mid");}}public static void func(int N, String from, String to, String other) {if (N == 1) { // baseSystem.out.println("Move 1 from " + from + " to " + to);} else {func(N - 1, from, other, to);System.out.println("Move " + N + " from " + from + " to " + to);func(N - 1, other, to, from);}}public static class Record {public boolean finish1;public int base;public String from;public String to;public String other;public Record(boolean f1, int b, String f, String t, String o) {finish1 = false;base = b;from = f;to = t;other = o;}}public static void hanoi3(int N) {if (N < 1) {return;}Stack<Record> stack = new Stack<>();stack.add(new Record(false, N, "left", "right", "mid"));while (!stack.isEmpty()) {Record cur = stack.pop();if (cur.base == 1) {System.out.println("Move 1 from " + cur.from + " to " + cur.to);if (!stack.isEmpty()) {stack.peek().finish1 = true;}} else {if (!cur.finish1) {stack.push(cur);stack.push(new Record(false, cur.base - 1, cur.from, cur.other, cur.to));} else {System.out.println("Move " + cur.base + " from " + cur.from + " to " + cur.to);stack.push(new Record(false, cur.base - 1, cur.other, cur.to, cur.from));}}}}public static void main(String[] args) {int n = 3;hanoi1(n);System.out.println("============");hanoi2(n);
//		System.out.println("============");
//		hanoi3(n);}

习题2 给你一个栈，请你逆序这个栈，不能申请额外的数据结构，只能使用递归函数。 如何实现?

public static void reverse(Stack<Integer> stack) {if (stack.isEmpty()) {return;}int i = f(stack);reverse(stack);stack.push(i);}// 栈底元素移除掉// 上面的元素盖下来// 返回移除掉的栈底元素public static int f(Stack<Integer> stack) {int result = stack.pop();if (stack.isEmpty()) {return result;} else {int last = f(stack);stack.push(result);return last;}}public static void main(String[] args) {Stack<Integer> test = new Stack<Integer>();test.push(1);test.push(2);test.push(3);test.push(4);test.push(5);reverse(test);while (!test.isEmpty()) {System.out.println(test.pop());}}

习题3 打印一个字符串的全部子序列，打印一个字符串的全部子序列，要求不要出现重复字面值的子序列

// s -> "abc" ->public static List<String> subs(String s) {char[] str = s.toCharArray();String path = "";List<String> ans = new ArrayList<>();process1(str, 0, ans, path);return ans;}// str 固定参数// 来到了str[index]字符，index是位置// str[0..index-1]已经走过了！之前的决定，都在path上// 之前的决定已经不能改变了，就是path// str[index....]还能决定，之前已经确定，而后面还能自由选择的话，// 把所有生成的子序列，放入到ans里去public static void process1(char[] str, int index, List<String> ans, String path) {if (index == str.length) {ans.add(path);return;}// 没有要index位置的字符process1(str, index + 1, ans, path);// 要了index位置的字符process1(str, index + 1, ans, path + String.valueOf(str[index]));}public static List<String> subsNoRepeat(String s) {char[] str = s.toCharArray();String path = "";HashSet<String> set = new HashSet<>();process2(str, 0, set, path);List<String> ans = new ArrayList<>();for (String cur : set) {ans.add(cur);}return ans;}public static void process2(char[] str, int index, HashSet<String> set, String path) {if (index == str.length) {set.add(path);return;}String no = path;process2(str, index + 1, set, no);String yes = path + String.valueOf(str[index]);process2(str, index + 1, set, yes);}public static void main(String[] args) {String test = "acccc";List<String> ans1 = subs(test);List<String> ans2 = subsNoRepeat(test);for (String str : ans1) {System.out.println(str);}System.out.println("=================");for (String str : ans2) {System.out.println(str);}System.out.println("=================");}

习题4 打印一个字符串的全部排列，要求不要出现重复的排列

public static List<String> permutation1(String s) {List<String> ans = new ArrayList<>();if (s == null || s.length() == 0) {return ans;}char[] str = s.toCharArray();ArrayList<Character> rest = new ArrayList<Character>();for (char cha : str) {rest.add(cha);}String path = "";f(rest, path, ans);return ans;}public static void f(ArrayList<Character> rest, String path, List<String> ans) {if (rest.isEmpty()) {ans.add(path);} else {int N = rest.size();for (int i = 0; i < N; i++) {char cur = rest.get(i);rest.remove(i);f(rest, path + cur, ans);rest.add(i, cur);}}}public static List<String> permutation2(String s) {List<String> ans = new ArrayList<>();if (s == null || s.length() == 0) {return ans;}char[] str = s.toCharArray();g1(str, 0, ans);return ans;}public static void g1(char[] str, int index, List<String> ans) {if (index == str.length) {ans.add(String.valueOf(str));} else {for (int i = index; i < str.length; i++) {swap(str, index, i);g1(str, index + 1, ans);swap(str, index, i);}}}public static List<String> permutation3(String s) {List<String> ans = new ArrayList<>();if (s == null || s.length() == 0) {return ans;}char[] str = s.toCharArray();g2(str, 0, ans);return ans;}public static void g2(char[] str, int index, List<String> ans) {if (index == str.length) {ans.add(String.valueOf(str));} else {boolean[] visited = new boolean[256];for (int i = index; i < str.length; i++) {if (!visited[str[i]]) {visited[str[i]] = true;swap(str, index, i);g2(str, index + 1, ans);swap(str, index, i);}}}}public static void swap(char[] chs, int i, int j) {char tmp = chs[i];chs[i] = chs[j];chs[j] = tmp;}public static void main(String[] args) {String s = "acc";List<String> ans1 = permutation1(s);for (String str : ans1) {System.out.println(str);}System.out.println("=======");List<String> ans2 = permutation2(s);for (String str : ans2) {System.out.println(str);}System.out.println("=======");List<String> ans3 = permutation3(s);for (String str : ans3) {System.out.println(str);}}