数组

class Solution {
 public void rotate(int[][] matrix) {
     if(matrix.length == 0 || matrix[0].length == 0){
         return;
     }
     //先上下翻转
     for(int i=0;i<matrix.length/2;i++){
         int[] temp = matrix[i];
         matrix[i] = matrix[matrix.length-i-1];
         matrix[matrix.length-i-1] = temp;
     }
     //对角翻转
     for(int i=0;i<matrix.length;i++){
         for(int j=0;j<i;j++){
             int temp = matrix[i][j];
             matrix[i][j] = matrix[j][i];
             matrix[j][i] = temp;
         }
     }
 }
}

LRU：https://leetcode-cn.com/problems/lru-cache/submissions/

旋转数组：https://leetcode-cn.com/problems/rotate-array/submissions/

class Solution {
 public void rotate(int[] nums, int k) {
     //k对len取余，得到最小的旋转次数
     k = k%nums.length;
     //先整个数组翻转一次
     swap(nums,0,nums.length-1);
     //翻转前k-1个
     swap(nums,0,k-1);
     //翻转剩余的
     swap(nums,k,nums.length-1);
 }

 public void swap(int[] nums,int left,int right){
     while(left < right){
         int temp = nums[left];
         nums[left] = nums[right];
         nums[right] = temp;
         left++;
         right--;
     }
 }
}

除自身外的乘积：https://leetcode-cn.com/problems/product-of-array-except-self/comments/

class Solution {
 public int[] productExceptSelf(int[] nums) {
     if(nums.length == 0){
         return new int[0];
     }
     int[] result = new int[nums.length];
     //初始化left为1，从左往右搜索，result不叠乘left
     int left = 1;
     for(int i=0;i<nums.length;i++){
         result[i] = left;
         left *= nums[i];
     }
     //初始化right为1，从右往左搜，result叠乘right
     int right = 1;
     for(int i=nums.length-1;i>=0;i--){
         result[i] *= right;
         right *= nums[i];
     }
     return result;
 }
}

找到所有消失的数：https://leetcode-cn.com/problems/find-all-numbers-disappeared-in-an-array/submissions/

class Solution {
 public List<Integer> findDisappearedNumbers(int[] nums) {
     List<Integer> result = new ArrayList<>();
     //nums[i]作为temp数组的索引，设置为1，然后遍历temp数组，temp[i]为0时，存入i到list
     int[] temp = new int[nums.length+1];
     for(int i=0;i<nums.length;i++){
         temp[nums[i]] = 1;
     }
     for(int i=1;i<temp.length;i++){
         if(temp[i] == 0){
             result.add(i);
         }
     }
     return result;
 }
}

有序矩阵中第K小的元素：https://leetcode-cn.com/problems/kth-smallest-element-in-a-sorted-matrix/submissions/

class Solution {
 public int kthSmallest(int[][] matrix, int k) {
     if(matrix.length == 0 || matrix[0].length == 0){
         return 0;
     }
     //降维
     int[] result = Merge(matrix,0,matrix.length-1);
     return result[k-1];
 }

 public int[] Merge(int[][] matrix,int start,int end){
     if(start == end){
         return matrix[start];
     }
     int middle = start + (end - start)/2;
     //分割成左右两个数组
     int[] left = Merge(matrix,start,middle);
     int[] right = Merge(matrix,middle+1,end);
     //合并左右数组
     return MergeSort(left,right);
 }

 public int[] MergeSort(int[] nums1,int[] nums2){
     int[] temp = new int[nums1.length + nums2.length];
     int index = 0;
     int index1 = 0;
     int index2 = 0;
     while(index1 < nums1.length && index2 < nums2.length){
         if(nums1[index1] < nums2[index2]){
             temp[index++] = nums1[index1++];
         }else{
             temp[index++] = nums2[index2++];
         }
     }
     while(index1 < nums1.length){
         temp[index++] = nums1[index1++];
     }
     while(index2 < nums2.length){
         temp[index++] = nums2[index2++];
     }
     return temp;
 }
}

把数组排成最小的数：https://leetcode-cn.com/problems/ba-shu-zu-pai-cheng-zui-xiao-de-shu-lcof/