力扣labuladong——一刷day23

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前言

我们不要每次都去一个字符一个字符地比较子串和模式串，而是维护一个滑动窗口，运用滑动哈希算法一边滑动一边计算窗口中字符串的哈希值，拿这个哈希值去和模式串的哈希值比较，这样就可以避免截取子串，从而把匹配算法降低为 O(N)，这就是 Rabin-Karp 指纹字符串查找算法的核心逻辑。

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一、力扣187. 重复的DNA序列

class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        List<String> res = new ArrayList<>();
        if(s.length() < 10)return res;
        Map<String,Integer> map = new HashMap<>();
        int left = 0, right = 9;
        while(right < s.length()){
            String cur = s.substring(left,right+1);
            right ++;
            map.put(cur,map.getOrDefault(cur,0)+1);
            left ++;
        }
        for(String k : map.keySet()){
            if(map.get(k) > 1){
                res.add(k);
            }
        }
        return res;
    }
}

`在滑动窗口中快速计算窗口中元素的哈希值，叫做滑动哈希技巧`

class Solution {
    public List<String> findRepeatedDnaSequences(String s) {
        int[] nums = new int[s.length()];
        for(int i = 0; i < s.length(); i ++){
            switch(s.charAt(i)){
                case 'A':
                    nums[i] = 0;break;
                case 'C' :
                    nums[i] = 1;break;
                case 'G' :
                    nums[i] = 2;break;
                case 'T':
                    nums[i] = 3;break;
            }
        }
        HashSet<Integer> seen = new HashSet<>();
        HashSet<String> res = new HashSet<>();
        int R = 4;//进制
        int L = 10;//当前位数
        int RL = (int)Math.pow(R,L-1);
        int windowHash = 0;
        int left = 0, right = 0;
        while(right < nums.length){
            windowHash = windowHash * R + nums[right];
            right ++;
            if(right - left == L){
                if(seen.contains(windowHash)){
                    res.add(s.substring(left,right));
                }else{
                    seen.add(windowHash);
                }
                windowHash = windowHash - nums[left] * RL;
                left ++;
            }
            
        }
        return new LinkedList<>(res);
    }
}

二、力扣28. 找出字符串中第一个匹配项的下标

class Solution {
    public int strStr(String haystack, String needle) {
        int L = needle.length();
        int R = 256;
        long LR = 1;
        long Q = 1658598167;
        for(int i = 1; i <= L-1; i ++){
            LR = (LR * R)%Q;
        }
        long needleHash = 0;
        long windowHash = 0;
        for(int i = 0; i < L; i ++){
            needleHash = (needleHash * R + needle.charAt(i))%Q ;
        }
        int left = 0, right = 0;
        while(right < haystack.length()){
            windowHash = ((windowHash * R)%Q + haystack.charAt(right))%Q;
            right ++;
            if(right - left == L){
                if(windowHash == needleHash){
                    if(needle.equals(haystack.substring(left,right))){
                        return left;
                    }
                }
                windowHash = (windowHash - (haystack.charAt(left)*LR)%Q+Q)%Q;
                left ++;
            }
        }
        return -1;
    }
}