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package class25;
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import java.util.ArrayList;
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import java.util.HashMap;
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import java.util.List;
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// 本题测试链接 : https://leetcode.com/problems/ip-to-cidr/
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public class Code01_IPToCIDR {
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public static List<String> ipToCIDR(String ip, int n) {
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// ip -> 32位整数
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int cur = status(ip);
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// cur这个状态，最右侧的1，能表示下2的几次方
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int maxPower = 0;
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// 已经解决了多少ip了
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int solved = 0;
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// 临时变量
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int power = 0;
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List<String> ans = new ArrayList<>();
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while (n > 0) {
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// cur最右侧的1，能搞定2的几次方个ip
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// cur : 000...000 01001000
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// 3
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maxPower = mostRightPower(cur);
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// cur : 0000....0000 00001000 -> 2的3次方的问题
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// sol : 0000....0000 00000001 -> 1 2的0次方
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// sol : 0000....0000 00000010 -> 2 2的1次方
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// sol : 0000....0000 00000100 -> 4 2的2次方
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// sol : 0000....0000 00001000 -> 8 2的3次方
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solved = 1;
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power = 0;
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// 怕溢出
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// solved
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while ((solved << 1) <= n && (power + 1) <= maxPower) {
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solved <<= 1;
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power++;
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}
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ans.add(content(cur, power));
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n -= solved;
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cur += solved;
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}
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return ans;
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}
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// ip -> int(32位状态)
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public static int status(String ip) {
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int ans = 0;
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int move = 24;
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for (String str : ip.split("\\.")) {
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// 17.23.16.5 "17" "23" "16" "5"
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// "17" -> 17 << 24
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// "23" -> 23 << 16
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// "16" -> 16 << 8
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// "5" -> 5 << 0
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ans |= Integer.valueOf(str) << move;
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move -= 8;
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}
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return ans;
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}
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public static HashMap<Integer, Integer> map = new HashMap<>();
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// 1 000000....000000 -> 2的32次方
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public static int mostRightPower(int num) {
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// map只会生成1次，以后直接用
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if (map.isEmpty()) {
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map.put(0, 32);
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for (int i = 0; i < 32; i++) {
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// 00...0000 00000001 2的0次方
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// 00...0000 00000010 2的1次方
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// 00...0000 00000100 2的2次方
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// 00...0000 00001000 2的3次方
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map.put(1 << i, i);
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}
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}
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// num & (-num) -> num & (~num+1) -> 提取出最右侧的1
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return map.get(num & (-num));
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}
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public static String content(int status, int power) {
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StringBuilder builder = new StringBuilder();
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for (int move = 24; move >= 0; move -= 8) {
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builder.append(((status & (255 << move)) >>> move) + ".");
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}
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builder.setCharAt(builder.length() - 1, '/');
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builder.append(32 - power);
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return builder.toString();
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}
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}
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