leetcode-數組篇

Remove Element

public class Lc27 {
    public static int removeElement(int[] nums, int val) {

        if (nums == null || nums.length == 0) {
            return 0;
        }

        int count = 0;
        for (int i = 0; i < nums.length;) {
            if (nums[i] == val) {
                count++;
                move(nums, i);
            } else {
                i++;
            }
        }
        return nums.length - count;
    }

    public static void move(int[] nums, int position) {
        for (int i = position; i < nums.length - 1; i++) {
            nums[i] = nums[i + 1];
        }
        nums[nums.length - 1] = Integer.MAX_VALUE;
    }

    public static void main(String[] args) {
        int[] nums = { 0, 1, 2, 2, 3, 0, 4, 2 };
        int val = 2;
        System.out.println(removeElement(nums, val));
    }
}

Remove Duplicates from Sorted Array，这里用了六种排序，由于题目中有负数导致基数排序可能会出现数组下标错误

import java.util.ArrayList;
import java.util.List;

/**
 * 俩个排序算法
 *
 */
public class Lc26 {

    public static int removeDuplicates(int[] nums) {
        int count = 0;
        for (int i = 0; i < nums.length - 1; i++) {
            if (nums[i] == nums[i + 1]) {
                nums[i] = Integer.MAX_VALUE;
                count++;
            }
        }
//        nums = insertSort(nums);
//        nums = shellSort(nums);
//        nums = selectSort(nums);
//        nums = bubbleSort(nums);
//        quickSort(nums, 0, nums.length - 1);
        nums = radixSort(nums);

        return nums.length - count;
    }

    /**
     * 插入排序，遍历所有的元素和以前排好的元素，如果选择的元素比以前的元素小就替换。
     */
    public static int[] insertSort(int[] nums) {
        for (int i = 1; i < nums.length; i++) {
            for (int j = 0; j < i; j++) {
                if (nums[i] < nums[j]) {
                    int temp = nums[i];
                    nums[i] = nums[j];
                    nums[j] = temp;
                    break;
                }
            }
        }
        return nums;
    }

    /**
     * 插入排序，遍历所有的元素和以前排好的元素，如果选择的元素比以前的元素小就替换。 希尔排序：在插入排序的基础上增加控制增量，逻辑分组数据，每次比较俩端数据
     */
    public static int[] shellSort(int[] nums) {
        for (int gap = nums.length / 2; gap > 0; gap /= 2) {
            for (int i = gap; i < nums.length; i++) {
                for (int j = i; j >= gap; j -= gap) {
                    if (nums[j] < nums[j - gap]) {
                        int temp = nums[j];
                        nums[j] = nums[j - gap];
                        nums[j - gap] = temp;
                    }
                }
            }
        }
        return nums;
    }

    /**
     * 选择排序：再要排序中的数组中，选出最小的数字和第一个数字替换，一次选出第二小的数组和第二个数字替换，一次类推
     */
    public static int[] selectSort(int[] nums) {
        for (int i = 0; i < nums.length; i++) {
            int index = nums[i];
            for (int j = i + 1; j < nums.length; j++) {
                if (nums[j] < nums[i]) {
                    int temp = nums[j];
                    nums[j] = nums[i];
                    nums[i] = temp;
                }
            }
        }
        return nums;
    }

    /*
     * 冒泡排序:对当前还未排好顺序的元素进行自顶向下排序，交换相邻的俩个数字，小数上浮，大数下沉
     */
    public static int[] bubbleSort(int[] nums) {
        for (int i = 0; i < nums.length; i++) {
            for (int j = i + 1; j < nums.length; j++) {
                if (nums[j] < nums[i]) {
                    int temp = nums[j];
                    nums[j] = nums[i];
                    nums[i] = temp;
                }
            }
        }
        return nums;
    }

    /**
     * 快速排序：选取一个基准值，利用二分法对其排序
     */
    public static void quickSort(int[] nums, int low, int hign) {
        if (low < hign) {
            int index = getIndex(nums, low, hign);
            quickSort(nums, 0, index - 1);
            quickSort(nums, index + 1, hign);
        }
    }

    private static int getIndex(int[] nums, int low, int hign) {
        int temp = nums[low];
        while (low < hign) {
            while (low < hign && nums[hign] >= temp) {
                hign--;
            }
            nums[low] = nums[hign];

            while (low < hign && nums[low] <= temp) {
                low++;
            }
            nums[hign] = nums[low];
        }
        nums[low] = temp;
        return low;
    }

    /**
     * 基数排序：低位优先，比较每个数字的低位，依次到高位,注意是正整数
     */
    public static int[] radixSort(int[] nums) {
        if (nums == null || nums.length == 0) {
            return nums;
        }
        int max = nums[0];
        for (int i = 0; i < nums.length; i++) {
            if (max < nums[i]) {
                max = nums[i];
            }
        }

        int digit = 0;
        while (max != 0) {
            max /= 10;
            digit++;
        }

        // init list
        List<List<Integer>> buckets = new ArrayList<List<Integer>>();
        for (int i = 0; i < 10; i++) {
            buckets.add(new ArrayList<>());
        }

        for (int i = 0; i < digit; i++) {
            for (int j = 0; j < nums.length; j++) {
                int key = (int) (nums[j] % Math.pow(10, i + 1) / Math.pow(10, i));
                buckets.get(key).add(nums[j]);
            }
            int count = 0;
            for (int j = 0; j < nums.length; j++) {

                while (buckets.get(j).size() > 0) {
                    nums[count++] = buckets.get(j).remove(0);
                }
            }
            // 分配完之后，将桶中的元素依次复制回数组
        }
        return nums;
    }

    public static void main(String[] args) {
        int[] nums = { -1, 0, 0, 0, 0, 3, 3 };
        System.out.println(removeDuplicates(nums));
    }

}

Remove Duplicates from Sorted Array II

import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

public class Lc80 {
    public static int removeDuplicates(int[] nums) {

        // map(数字/次数)
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        int count = 0;
        for (int i = 0; i < nums.length; i++) {
            if (map.containsKey(nums[i])) {
                int temp = map.get(nums[i]);
                if (temp >= 2) {
                    count++;
                    nums[i] = Integer.MAX_VALUE;
                } else {
                    map.put(nums[i], ++temp);
                }
            } else {
                map.put(nums[i], 1);
            }
        }

        Arrays.sort(nums);
        return nums.length - count;
    }

    public static void main(String[] args) {
        int[] nums = { 1, 1, 1 };
        System.out.println(removeDuplicates(nums));
    }
}

Find the Celebrity

Rotate Array

public class Lc189 {
    public static void rotate(int[] nums, int k) {
        int previous = 0;
        for (int i = 0; i < k; i++) {
            previous = nums[nums.length - 1];
            for (int j = 0; j < nums.length; j++) {
                int temp = previous;
                previous = nums[j];
                nums[j] = temp;
            }
        }
    }

    public static void main(String[] args) {
        int[] nums = { 1, 2, 3, 4, 5, 6, 7 };
        int k = 3;
        rotate(nums, k);
    }

}

First Missing Positive

import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

public class Lc41 {
    public static int firstMissingPositive(int[] nums) {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < nums.length; i++) {
            if (map.containsKey(nums[i])) {
                nums[i] = Integer.MAX_VALUE;
            } else {
                map.put(nums[i], i);
            }
        }

        Arrays.sort(nums);
        int min = 0;
        int minPosition = 0;
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] > 0) {
                minPosition = i;
                min = nums[i];
                break;
            }
        }
        if (min != 1) {
            return 1;
        }

        int j = 1;
        for (int i = minPosition; i < nums.length; i++, j++) {
            if (nums[i] != j) {
                return j;
            }
        }
        return j;

    }

    public static void move(int[] nums, int position) {
        for (int i = 0; i < nums.length - 1; i++) {
            nums[i] = nums[i + 1];
        }
        nums[nums.length - 1] = Integer.MAX_VALUE;
    }

    public static void main(String[] args) {
        int[] nums = { 0, 2, 2, 1, 1 };
        System.out.println(firstMissingPositive(nums));
    }
}

Bulls and Cows

import java.util.ArrayList;
import java.util.List;

public class Lc229 {
    public static String getHint(String secret, String guess) {
        int bulls = 0;
        int cows = 0;
        List<String> secretDigits = convertToAscall(secret);
        List<String> guessDigits = convertToAscall(guess);
        for (int i = 0; i < secretDigits.size(); i++) {
            if (secretDigits.get(i).equals(guessDigits.get(i))) {
                bulls++;
            } else {
                cows++;
            }
        }
        return bulls + "A" + cows + "B";

    }

    private static List<String> convertToAscall(String s) {
        StringBuffer sb = new StringBuffer();
        char[] chars = s.toCharArray();
        for (char c : chars) {
            sb.append((int) c).append(",");
        }
        String[] str = sb.toString().split(",");
        List<String> lists = new ArrayList<>();
        for (String string : str) {
            lists.add(string);
        }

        return lists;
    }

    public static void main(String[] args) {
        String secret = "1123";
        String guess = "0111";
        System.out.println(getHint(secret, guess));
    }
}

参考文档：https://cspiration.com/leetcodeClassification#10309