leetcode hashmap

187. Repeated DNA Sequences

求重复的DNA序列

public List<String> findRepeatedDnaSequences(String s) {
    Set seen = new HashSet(), repeated = new HashSet();
    for (int i = 0; i + 9 < s.length(); i++) {
        String ten = s.substring(i, i + 10);
        if (!seen.add(ten))
            repeated.add(ten);
    }
    return new ArrayList(repeated);
}

hashset   !seen.add(ten) 添加失败返回false

299 Bulls and Cows

有一个四位数字，你猜一个结果，然后根据你猜的结果和真实结果做对比，提示有多少个数字和位置都正确的叫做bulls，还提示有多少数字正确但位置不对的叫做cows，根据这些信息来引导我们继续猜测正确的数字。这道题并没有让我们实现整个游戏，而只用实现一次比较即可。给出两个字符串，让我们找出分别几个bulls和cows。这题需要用哈希表，来建立数字和其出现次数的映射。我最开始想的方法是用两次遍历，第一次遍历找出所有位置相同且值相同的数字，即bulls，并且记录secret中不是bulls的数字出现的次数。然后第二次遍历我们针对guess中不是bulls的位置，如果在哈希表中存在，cows自增1，然后映射值减1，参见如下代码：

class Solution {
public:
    string getHint(string secret, string guess) {
        int m[256] = {0}, bulls = 0, cows = 0;
        for (int i = 0; i < secret.size(); ++i) {
            if (secret[i] == guess[i]) ++bulls;
            else ++m[secret[i]];
        }
        for (int i = 0; i < secret.size(); ++i) {
            if (secret[i] != guess[i] && m[guess[i]]) {
                ++cows;
                --m[guess[i]];
            }
        }
        return to_string(bulls) + "A" + to_string(cows) + "B";
    }
};

我们其实可以用一次循环就搞定的，在处理不是bulls的位置时，我们看如果secret当前位置数字的映射值小于0，则表示其在guess中出现过，cows自增1，然后映射值加1，如果guess当前位置的数字的映射值大于0，则表示其在secret中出现过，cows自增1，然后映射值减1，参见代码如下：

public String getHint(String secret, String guess) {
    int bulls = 0;
    int cows = 0;
    int[] numbers = new int[10];
    for (int i = 0; i<secret.length(); i++) {
        int s = Character.getNumericValue(secret.charAt(i));
        int g = Character.getNumericValue(guess.charAt(i));
        if (s == g) bulls++;
        else {
            if (numbers[s] < 0) cows++;
            if (numbers[g] > 0) cows++;
            numbers[s] ++;
            numbers[g] --;
        }
    }
    return bulls + "A" + cows + "B";
}

49. Group Anagrams

hashmap操作

public class Solution {
    public List<List<String>> groupAnagrams(String[] strs) {
        if (strs == null || strs.length == 0) return new ArrayList<List<String>>();
        Map<String, List<String>> map = new HashMap<String, List<String>>();
        for (String s : strs) {
            char[] ca = s.toCharArray();
            Arrays.sort(ca);
            String keyStr = String.valueOf(ca);
            if (!map.containsKey(keyStr)) map.put(keyStr, new ArrayList<String>());
            map.get(keyStr).add(s);
        }
        return new ArrayList<List<String>>(map.values());
    }
}

743. Network Delay Time

1. Use Map<Integer, Map<Integer, Integer>> to store the source node, target node and the distance between them.
2. Offer the node K to a PriorityQueue.
3. Then keep getting the closest nodes to the current node and calculate the distance from the source (K) to this node (absolute distance). Use a Map to store the shortest absolute distance of each node.
4. Return the node with the largest absolute distance.

public int networkDelayTime(int[][] times, int N, int K) {
        if(times == null || times.length == 0){
            return -1;
        }
        // store the source node as key. The value is another map of the neighbor nodes and distance.
        Map<Integer, Map<Integer, Integer>> path = new HashMap<>();
        for(int[] time : times){
            Map<Integer, Integer> sourceMap = path.get(time[0]);
            if(sourceMap == null){
                sourceMap = new HashMap<>();
                path.put(time[0], sourceMap);
            }
            Integer dis = sourceMap.get(time[1]);
            if(dis == null || dis > time[2]){
                sourceMap.put(time[1], time[2]);
            }
            
        }

        //Use PriorityQueue to get the node with shortest absolute distance 
        //and calculate the absolute distance of its neighbor nodes.
        Map<Integer, Integer> distanceMap = new HashMap<>();
        distanceMap.put(K, 0);
        PriorityQueue<int[]> pq = new PriorityQueue<>((i1, i2) -> {return i1[1] - i2[1];});
        pq.offer(new int[]{K, 0});
        int max = -1;
        while(!pq.isEmpty()){
            int[] cur = pq.poll();
            int node = cur[0];
            int distance = cur[1];

            // Ignore processed nodes
            if(distanceMap.containsKey(node) && distanceMap.get(node) < distance){
                continue;
            }
            
            Map<Integer, Integer> sourceMap = path.get(node);
            if(sourceMap == null){
                continue;
            }
            for(Map.Entry<Integer, Integer> entry : sourceMap.entrySet()){
                int absoluteDistence = distance + entry.getValue();
                int targetNode = entry.getKey();
                if(distanceMap.containsKey(targetNode) && distanceMap.get(targetNode) <= absoluteDistence){
                    continue;
                }
                distanceMap.put(targetNode, absoluteDistence);
                pq.offer(new int[]{targetNode, absoluteDistence});
            }
        }
        // get the largest absolute distance.
        for(int val : distanceMap.values()){
            if(val > max){
                max = val;
            }
        }
        return distanceMap.size() == N ? max : -1;
}