【LEETCODE】57、数组分类,适中级别,题目:969、442、695
2024-10-21 05:48:46
package y2019.Algorithm.array.medium; import java.util.ArrayList;
import java.util.List; /**
* @ProjectName: cutter-point
* @Package: y2019.Algorithm.array.medium
* @ClassName: PancakeSort
* @Author: xiaof
* @Description: TODO 969. Pancake Sorting
* Given an array A, we can perform a pancake flip: We choose some positive integer k <= A.length,
* then reverse the order of the first k elements of A.
* We want to perform zero or more pancake flips (doing them one after another in succession) to sort the array A.
*
* Return the k-values corresponding to a sequence of pancake flips that sort A.
* Any valid answer that sorts the array within 10 * A.length flips will be judged as correct.
*
* Input: [3,2,4,1]
* Output: [4,2,4,3]
* Explanation:
* We perform 4 pancake flips, with k values 4, 2, 4, and 3.
* Starting state: A = [3, 2, 4, 1]
* After 1st flip (k=4): A = [1, 4, 2, 3]
* After 2nd flip (k=2): A = [4, 1, 2, 3]
* After 3rd flip (k=4): A = [3, 2, 1, 4]
* After 4th flip (k=3): A = [1, 2, 3, 4], which is sorted.
*
* 参考:https://blog.csdn.net/fuxuemingzhu/article/details/85937314
*
* @Date: 2019/7/16 8:57
* @Version: 1.0
*/
public class PancakeSort { public List<Integer> solution(int[] A) {
//思路是这样的,就是每次吧最大的做一个翻转,移动到最前面,然后再翻转到最后面,这样每次都可以从数据中排除掉最大的那个
//但是由于这个题的数字都是按照1~n的顺序给的值,那么就不需要每次都取最大值,只要取index索引就可以了
List<Integer> res = new ArrayList<>();
for(int i = A.length, x; i > 0; --i) {
//寻找最大的位置
for(x = 0; A[x] != i; ++x);
//当x所在的索引跟当前应该的最大值相等的时候,也就是x指向了最大值的位置的-1位置,我们翻转两次
//第一次吧值翻转到最前面,第二次翻转到最后面
reverse(A, x);
res.add(x + 1);
//然后翻转到对应的位置
reverse(A, i - 1);
res.add(i);
} return res;
} private void reverse(int[] A, int k) {
//翻转k位
for(int i = 0, j = k; i < j; ++i,--j) {
//前后交换
int temp = A[i];
A[i] = A[j];
A[j] = temp;
}
} public static void main(String[] args) {
int data[] = {3,2,4,1};
PancakeSort fuc = new PancakeSort();
System.out.println(fuc.solution(data));
System.out.println();
} }
package y2019.Algorithm.array.medium; import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Set; /**
* @ProjectName: cutter-point
* @Package: y2019.Algorithm.array.medium
* @ClassName: FindDuplicates
* @Author: xiaof
* @Description: TODO 442. Find All Duplicates in an Array
* Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.
* Find all the elements that appear twice in this array.
* Could you do it without extra space and in O(n) runtime?
*
* Input:
* [4,3,2,7,8,2,3,1]
* Output:
* [2,3]
*
* 给定一个整数数组 a,其中1 ≤ a[i] ≤ n (n为数组长度), 其中有些元素出现两次而其他元素出现一次。
* 找到所有出现两次的元素。
* 你可以不用到任何额外空间并在O(n)时间复杂度内解决这个问题吗?
* @Date: 2019/7/16 9:00
* @Version: 1.0
*/
public class FindDuplicates { public List<Integer> solution(int[] nums) { //直接用set
List<Integer> res = new ArrayList<>();
if(nums == null || nums.length <= 0) {
return res;
}
//受限还是排序
Arrays.sort(nums);
//遍历一次
int preValue = nums[0];
for(int i = 1; i < nums.length; ++i) {
int curValue = nums[i];
if(preValue == curValue) {
res.add(curValue);
} else {
preValue = curValue;
}
} return res;
}
}
package y2019.Algorithm.array.medium; /**
* @ProjectName: cutter-point
* @Package: y2019.Algorithm.array.medium
* @ClassName: MaxAreaOfIsland
* @Author: xiaof
* @Description: TODO 695. Max Area of Island
* Given a non-empty 2D array grid of 0's and 1's, an island is a group of 1's (representing land)
* connected 4-directionally (horizontal or vertical.) You may assume all four edges of the grid are surrounded by water.
*Find the maximum area of an island in the given 2D array. (If there is no island, the maximum area is 0.)
*
* [[0,0,1,0,0,0,0,1,0,0,0,0,0],
* [0,0,0,0,0,0,0,1,1,1,0,0,0],
* [0,1,1,0,1,0,0,0,0,0,0,0,0],
* [0,1,0,0,1,1,0,0,1,0,1,0,0],
* [0,1,0,0,1,1,0,0,1,1,1,0,0],
* [0,0,0,0,0,0,0,0,0,0,1,0,0],
* [0,0,0,0,0,0,0,1,1,1,0,0,0],
* [0,0,0,0,0,0,0,1,1,0,0,0,0]]
* Given the above grid, return 6. Note the answer is not 11, because the island must be connected 4-directionally.
*
* 给定一个包含了一些 0 和 1的非空二维数组 grid , 一个 岛屿 是由四个方向 (水平或垂直) 的 1 (代表土地) 构成的组合。你可以假设二维矩阵的四个边缘都被水包围着。
* 找到给定的二维数组中最大的岛屿面积。(如果没有岛屿,则返回面积为0。)
* 来源:力扣(LeetCode)
* 链接:https://leetcode-cn.com/problems/max-area-of-island
* 著作权归领扣网络所有。商业转载请联系官方授权,非商业转载请注明出处。
*
* @Date: 2019/7/16 9:00
* @Version: 1.0
*/
public class MaxAreaOfIsland { public int solution(int[][] grid) {
//寻找聚集度最高的和,遍历所有的岛屿,然后对附近的所有1求和,每次求和探索四个位置的和,上下左右
int maxIsland = 0;
for(int i = 0; i < grid.length; ++i) {
for(int j = 0; j < grid[i].length; ++j) {
//求出最大的岛屿
maxIsland = Math.max(maxIsland, areaOfIsLand(grid, i, j));
}
}
return maxIsland;
} private int areaOfIsLand(int[][] grid, int x, int y) {
if(x >= 0 && x < grid.length && y >=0 && y < grid[x].length && grid[x][y] == 1) {
//设置标记
grid[x][y] &= 0;
return 1 + areaOfIsLand(grid, x - 1, y) + areaOfIsLand(grid, x + 1, y) + areaOfIsLand(grid, x, y - 1) + areaOfIsLand(grid, x, y + 1);
} return 0;
} }
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