1、两个数组的交集

import java.util.*;
class Solution {
    public int[] intersection(int[] nums1, int[] nums2) {
        if(nums1.length == 0 || nums2.length == 0 || nums1 == null || nums2 == null){
            return new int[0];
        }
         Set<Integer> s = new HashSet<>();
         Set<Integer> s2 = new HashSet<>();
         for(int i : nums1){
             s.add(i);
         }
         for(int i : nums2){
             if(s.contains(i)){
                s2.add(i);
             }
         }
         int[] res = new int[s2.size()];
         int j = 0;
         for(Integer i : s2){
             res[j++] = i;
         }
         return res;
    }
}

2、两个数组的交集 II

class Solution {
    public int[] intersect(int[] nums1, int[] nums2) {
        if (nums1.length > nums2.length) {
            int[] temp = nums1;
            nums1 = nums2;
            nums2 = temp;
        }
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int num : nums1) {
            int count = map.getOrDefault(num, 0) + 1;
            map.put(num, count);
        }
        int[] res = new int[nums1.length];
        int index = 0;
        for (int num : nums2) {
            int count = map.getOrDefault(num, 0);
            if (count > 0) {
                res[index++] = num;
                count--;
                if (count > 0) {
                    map.put(num, count);
                } else {
                    map.remove(num);
                }
            }
        }
        return Arrays.copyOfRange(res, 0, index);
    }
}

3、在排序数组中查找元素的第一个和最后一个位置

class Solution {
    public int[] searchRange(int[] nums, int target) {
        int[] res = new int[2];
        if (nums.length == 0 || nums == null){
            res[0] = -1;
            res[1] = -1;
            return res;
        }
        int left = 0;
        int right = nums.length - 1;
        res[0] = -1;
        res[1] = -1;
        while(left <= right){
            int mid = left + (right - left) / 2;
            if (nums[mid] == target){
                res[0] = mid;
                right = mid - 1;
            } else if (nums[mid] < target) {
                left = mid + 1;
            }else {
                right = mid - 1;
            }
        }

        left = 0;
        right = nums.length - 1;
        while(left <= right){
            int mid = left + (right - left) / 2;
            if (nums[mid] == target){
                res[1] = mid;
                left = mid + 1;
            } else if (nums[mid] < target) {
                left = mid + 1;
            }else {
                right = mid - 1;
            }
        }
        return res;
    }
}

4、在排序数组中查找数字 I

class Solution {
    public int search(int[] nums, int target) {
        if(nums == null && nums.length == 0){
            return 0;
        }
        int[] ans = new int[2];
        ans[0] = -1;
        ans[1] = -1;
        int left = 0;
        int right = nums.length - 1;
        while(left <= right){
            int mid = left + (right - left) / 2;
            if (nums[mid] == target){
                ans[0] = mid;
                right = mid - 1;
            } else if (nums[mid] < target) {
                left = mid + 1;
            }else {
                right = mid - 1;
            }
        }

        left = 0;
        right = nums.length - 1;
        while(left <= right){
            int mid = left + (right - left) / 2;
            if (nums[mid] == target){
                ans[1] = mid;
                left = mid + 1;
            } else if (nums[mid] < target) {
                left = mid + 1;
            }else {
                right = mid - 1;
            }
        }
        if(ans[0] == -1 && ans[1] == -1){
            return 0;
        }
        return ans[1] - ans[0] + 1;
    }
}

5、数组中的第K个最大元素

class Solution {
    public int findKthLargest(int[] nums, int k) {
        return sortK(nums,0,nums.length - 1,nums.length - k);
    }
    private static int sortK(int[] nums, int left, int right, int k) {
        if (left < right){
            int i = left;
            int j = right;
            int pivot = nums[left];
            while (i < j){
                while (i < j && nums[j] >= pivot)
                {
                    j--;
                }
                swap(nums,i,j);
                while (i < j && nums[i] <= pivot){
                    i++;
                }
                swap(nums,i,j);
            }
            if(i > k){
                sortK(nums,left,i - 1,k);                
            }else if(i < k){
                sortK(nums,i + 1,right,k);                
            }
        }
        return nums[k];
    }

    public static void  swap(int[] nums,int i, int j){
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}

6、前 K 个高频元素

import java.util.*;
class Solution {
    public int[] topKFrequent(int[] nums, int k) {
        int[] res = new int[k];
        HashMap<Integer,Integer> map = new HashMap<>();
        for(int num : nums){
            map.put(num,map.getOrDefault(num,0) + 1);
        }

        Set<Map.Entry<Integer,Integer>> set = map.entrySet();
        PriorityQueue<Map.Entry<Integer,Integer>> queue = new PriorityQueue<>((o1,o2) ->{return o1.getValue() - o2.getValue();});
        for(Map.Entry<Integer,Integer> entry: set){
            queue.offer(entry);
            if(queue.size() > k){
                queue.poll();
            }
        }
        for(int i = k - 1; i>=0; i--){
            res[i] = queue.poll().getKey();
        }
        return res;
    }
}

7、零矩阵

class Main {
    public void main(int[][] matrix) {
        if(matrix == null){
            return;
        }
        int[] m  = new int[matrix.length];
        int[] n = new int[matrix[0].length];
        for(int i = 0; i < matrix.length; i++){
            for(int j = 0; j < matrix[0].length; j++){
                if(matrix[i][j] == 0){
                    m[i] = 1;
                    n[j] = 1;
                }
            }
        }

        for(int i = 0; i < matrix.length; i++){
            for(int j = 0; j < matrix[0].length; j++){
                if(m[i] == 1 || n[j] == 1){
                    matrix[i][j] = 0;
                }
            }
        }
    }
}

8、螺旋矩阵

import java.util.*;

class Main {
    public List<Integer> main(int[][] matrix) {
        if(matrix == null || matrix.length == 0){
            return null;
        }
        List<Integer> res = new ArrayList<>();
        int left = 0;
        int right = matrix[0].length - 1;
        int top = 0;
        int bottom = matrix.length - 1;
        while (left < right && top < bottom) {
            for (int i = left; i < right; i++) {
                res.add(matrix[top][i]);
            }
            for (int i = top; i < bottom; i++) {
                res.add(matrix[i][right]);
            }
            for (int i = right; i > left; i--) {
                res.add(matrix[bottom][i]);
            }
            for (int i = bottom; i > top; i--) {
                res.add(matrix[i][left]);
            }
            left++;
            right--;
            bottom--;
            top++;
        }
        if (bottom == top) {
            for (int i = left; i <= right; i++) {
                res.add(matrix[top][i]);
            }
        }else if(left == right){
            for (int i = top; i <= bottom; i++) {
                res.add(matrix[i][right]);
            }
        }

        return res;
    }
}

9、螺旋矩阵 II

class Solution {
    public int[][] generateMatrix(int n) {
        if( n == 0){
            return null;
        }
        int[][] matrix = new int[n][n];
        int left = 0;
        int right = matrix[0].length - 1;
        int top = 0;
        int bottom = matrix.length - 1;
        int sum = 1;
        while (left < right && top < bottom) {
            for (int i = left; i < right; i++) {
                matrix[top][i] = sum++;
            }
            for (int i = top; i < bottom; i++) {
                matrix[i][right] = sum++;
            }
            for (int i = right; i > left; i--) {
                matrix[bottom][i] = sum++;
            }

            for (int i = bottom; i > top; i--) {
                matrix[i][left] = sum++;
            }
            left++;
            top++;
            bottom--;
            right--;
        }
        if (top == bottom) {
            for (int i = left; i <= right; i++) {
                matrix[top][i] = sum++;
            }
        } else if (left == right) {
            for (int i = top; i <= bottom; i++) {
                matrix[i][right] = sum++;
            }
        }
        return matrix;
    }
}

10、旋转图像

class Solution {
    public void rotate(int[][] matrix) {
        int n = matrix.length;
        for (int i = 0; i < n / 2; i++) {
            for (int j = 0; j < (n + 1) / 2; j++) {
                int tmp = matrix[i][j];
                matrix[i][j] = matrix[n - 1 - j][i];
                matrix[n - 1 - j][i] = matrix[n - 1 - i][n - 1 - j];
                matrix[n - 1 - i][n - 1 - j] = matrix[j][n - 1 - i];
                matrix[j][n - 1 - i] = tmp;
            }
        }
    }
}