1.有效的字母异位词

class Solution {
    public boolean isAnagram(String s, String t) {
        if(s.length()!=t.length()) return false;
        int []res = new int[26];
        for(int i=0;i<s.length();i++){
            res[s.charAt(i)-'a']++;
        }
        for(int i=0;i<t.length();i++){
            res[t.charAt(i)-'a']--;
            if(res[t.charAt(i)-'a']<0) return false;
        }

        return true;


    }
}

2.两数组交集

class Solution {
   public int[] intersection(int[] nums1, int[] nums2) {
        Map<Integer,Integer> hashMap = new HashMap<>();
        boolean flag = false;
        for(int x:nums1){
            if(!hashMap.containsKey(x)) hashMap.put(x,1);
            else hashMap.put(x,hashMap.get(x)+1);
        }
        List<Integer>list = new LinkedList<>();
        for(int x:nums2){
            if(hashMap.containsKey(x)) {
                list.add(x);
                flag = true;
            }
        }
        Set<Integer>set = new HashSet<>(list);
        if(flag==false) return new int[0];
        return set.stream().mapToInt(i -> i).toArray();

    }
}

3.快乐数

class Solution {
    public int getNext(int n) {
        int sum = 0;
        while (n != 0) {
            int res = n % 10;
            sum += res * res;
            n /= 10;
        }
        return sum;

    }

    public boolean isHappy(int n) {
        HashSet<Integer> set = new HashSet<>();
        while (n != 1 && set.contains(n)==false) {
            set.add(n);
            n = getNext(n);
        }
        if (n != 1)
            return false;
        return true;
    }
}

4.两数之和

class Solution {
    public int[] twoSum(int[] nums, int target) {
            Map<Integer,Integer>map = new HashMap<>();
            for(int i=0;i<nums.length;i++){
                if(!map.containsKey(nums[i])) map.put(nums[i],i);
            }
            for(int i=0;i<nums.length;i++){
                int x = nums[i];
                if(map.containsKey(target-x)&&map.get(target-x)!=i){
                    return new int[]{i,map.get(target-x)};
                }
            }
            return null;
        }
}

5.四数相加

class Solution {
    public int fourSumCount(int[] nums1, int[] nums2, int[] nums3, int[] nums4) {
        int res = 0;
        Map<Integer,Integer>map = new HashMap<>();
        for(int x:nums1){
            for(int y:nums2){
                map.put(x+y,map.getOrDefault(x+y,0)+1);
            }
        }

        for(int x:nums3){
            for (int y:nums4){
                if(map.containsKey(0-x-y)) res+=map.getOrDefault(0-x-y,0);
            }


        }
        return  res;
    }
}

6.三数之和

class Solution {
        public List<List<Integer>> threeSum(int[] nums) {
            List<List<Integer>>res =  new LinkedList<>();
            Arrays.sort(nums);
            for(int i=0;i<nums.length;i++){
                if(nums[i]>0)  break;
                if(i>0&&nums[i]==nums[i-1]) continue;
                int l = i+1,r = nums.length-1;
                while (l<r){
                    int sum = nums[l]+nums[r]+nums[i];
                    if(sum<0){
                        l++;
                    }else if(sum>0){
                        r--;
                    }else {
                        res.add(new LinkedList<>(Arrays.asList(nums[l],nums[r],nums[i])));
                        l++;
                        r--;
                        while(nums[l]==nums[l-1]&&l<r) l++;
                        while (nums[r]==nums[r+1]&&l<r) r--;

                    }

                }

            }

            return res;
        }
}