第三章 基础算法和数据结构高频题 I
区间类问题
1 Missing Interval
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public List<String> findMissingRanges(int[] nums, int lower, int upper) { List<String> res = new ArrayList<>(); if (nums == null || nums.length == 0) { return res; } addRange(res, lower, (long)nums[0] - 1); for (int i = 1; i < nums.length; i++) { addRange(res, (long)nums[i - 1] + 1, (long)nums[i] - 1); } addRange(res, (long)nums[nums.length - 1] + 1, upper); return res; } void addRange(List<String> res, long l, long r) { if (l > r) { return; } if (l == r) { res.add(l + ""); return; } res.add(l + "->" + r); }
2 Merge intervals
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public List<Interval> merge(List<Interval> inte) { List<Interval> res = new ArrayList<>(); inte.sort(Comparator.comparing(i -> i.start)); Interval last = null; for (Interval item : inte) { if (last == null || last.end < item.start) { res.add(item); last = item; } else { last.end = Math.max(item.end, last.end); } } return res; }
3 Insert interval
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public ArrayList<Interval> insert(ArrayList<Interval> intervals, Interval newInterval) { ArrayList<Interval> res = new ArrayList<Interval>(); int id = 0; while (id < intervals.size() && intervals.get(id).start < newInterval.start) { id++; } intervals.add(id, newInterval); Interval last = null; for (Interval item : intervals) { if (last == null || last.end < item.start) { res.add(item); last = item; } else { last.end = Math.max(item.last, last.end); } } return res; }
Hash 字符/ 字符串统计类问题
4 First Position Unique Character
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public int firstUniqChar(String s) { // write your code here\ int[] num = new int[256]; for (char c : s.toCharArray()) { num[c]++; } for (int i = 0; i < s.length(); i++) { if (num[s.charAt(i)] == 1) { return i; } } return -1; }
5 Substring Anagrams
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public List<Integer> findAnagrams(String s, String p) { List<Integer> res = new ArrayList<>(); if (s.length() < p.length()) { return res; } char[] cs = s.toCharArray(); char[] ps = p.toCharArray(); int[] det = new int[256]; int absSum = 0; for (int i = 0; i < p.length(); i++) { det[cs[i]]++; det[ps[i]]--; } for (int i = 0; i < 256; i++) { absSum += Math.abs(det[i]); } if (absSum == 0) { res.add(0); } for (int i = p.length(); i < s.length(); i++) { char l = cs[i - ps.length]; char r = cs[i]; absSum = absSum - Math.abs(det[l]) - Math.abs(det[r]); det[l]--; det[r]++; absSum = absSum + Math.abs(det[l]) + Math.abs(det[r]); if (absSum == 0) { res.add(i - ps.length + 1); } } return res; }
6 Word Abbreviation Set
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class VaildWordAbbr { private Map<String, Integer> dict = new HashMap<>(); private Map<String, Integer> abbr = new HashMap<>(); public VaildWordAbbr(String[] dic) { for (String str : dic) { dict.put(str, dict.getOrDefault(str, 0) + 1); String abb = getAbbr(str); dict.put(abb, dict.getOrDefault(abb, 0) + 1); } } public boolean isUnique(String word) { String a = getAbbr(word); return dict.get(word) == abbr.get(a); } String getAbbr(String word) { if (word.length() < 3) { return word; } return "" + word.charAt(0) + (word.length() - 2) + word.charAt(word.length() - 1); }
7 Longest Consecutive Sequence
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public int longestConsecutive(int[] num) { // write you code here if (num == null || num.length == 0) { return 0; } HashSet<Integer> set = new HashSet<Integer>(); for (int i : num) { set.add(i); } int res = 0; for (int item : num) { if (set.contains(item)) { set.remove(item); int l = item - 1; int r = item + 1; while (set.contains(l)) { set.remove(l); l--; } while (set.contains(r)) { set.remove(r); r++; } res = Math.max(res, r - l - 1); } } return res; }
8 Load Balancer
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class LoadBalance { private List<Integer> list = new ArrayList<>(); private Map<Integer, Integer> map = new HashMap<>(); private int n = 0; private Random r = new Random(); public LoadBalance() { } public void add(int id) { if (!map.containsKey(id)) { list.add(id); map.put(n, id); n++; } } public void remove(int id) { if (map.containsKey(id)) { int lastItem = list.get(n - 1); int removeIndex = map.get(id); map.put(lastItem, removeIndex); list.set(removeIndex, lastItem); list.remove(n - 1); map.remove(id); n--; } } public int pick() { return list.get(r.nextInt(n)); } }