各种排序方法总结

Comparing Methods

a.compareTo(b); //按字典顺序比较两个字符串，返回0: a=b, 返回正数: a > b, 返回负数: a < b

 

Sorting Methods

Arrays.sort(); //字母序优先于长度
Collections.sort();

 

Custom Comparator

• max heap，按频率排序，频率相同则字典序

PriorityQueue<String> heap = new PriorityQueue<>((a, b) -> map.get(a) == map.get(b) ? b.compareTo(a) : (map.get(a) - map.get(b)));

 

 

HashMap, Heap... Related Sort

• Heap

PriorityQueue 默认小根堆（自动排序特性），不保证整个队列都有序，只保证队列头是最小的
PriorityQueue <Integer> pq = new PriorityQueue<> (26, Collections.reverseOrder()); //大根堆

• TreeMap documents

treemap基于红黑树，检索时间是O(logn)，默认按照key的自然大小排序（升序，从小到大）

• Map sort by value

List<Map.Entry<Integer, Integer>> list = new ArrayList<Map.Entry<Integer, Integer>>(map.entrySet());
Collections.sort(list, new Comparator<Map.Entry<Integer, Integer>>() {
    public int compare(Entry<Integer, Integer> o1, Entry<Integer, Integer> o2) {
        return o1.getValue().compareTo(o2.getValue());
    }
});
//    for (Map.Entry<Integer, Integer> mapping : list) {
//        System.out.println(mapping.getKey() + ":" + mapping.getValue());
//    }

 

 

经典排序

 

1. Selection Sort

public class Solution {
    public void sortIntegers(int[] A) {
        // write your code here
        for(int i = 0; i < A.length; i++) {
            int min_idx = i;
            for(int j = i + 1; j < A.length; j++) {
                if(A[j] < A[min_idx])
                    min_idx = j;
            }
            int tmp = A[i];
            A[i] = A[min_idx];
            A[min_idx] = tmp;
        }
    }
}

2. Merge Sort

public class MergeSort {
    public static int[] mergeSort(int[] array) {
        // Write your solution here
        if(array == null || array.length == 0) return array;
        int[] helper = new int[array.length];
        mergeSort(array, helper, 0, array.length - 1);
        return array;
    }
    private static void mergeSort(int[] array, int[] helper, int left, int right) {
        if(left >= right) return;
        int mid = left + (right - left) / 2;
        mergeSort(array, helper, left, mid);
        mergeSort(array, helper, mid + 1, right);
        merge(array, helper, left, mid, right);
    }
    private static void merge(int[] a, int[] helper, int left, int mid, int right) {
        for(int i = left; i <= right; i++) {
            helper[i] = a[i];
        }
        int p1 = left, p2 = mid + 1, p = left;
        while(p1 <= mid && p2 <= right) {
            if(helper[p1] <= helper[p2])
                a[p++] = helper[p1++];
            else
                a[p++] = helper[p2++];
        }
        while(p1 <= mid) {
            a[p++] = helper[p1++];
        }
        while(p2 <= right) {
            a[p++] = helper[p2++];
        }
    }

    public static void main(String[] args) {
        int[] arr = {3,1,5,7,9,8,4,2,0};
        MergeSort solution = new MergeSort();
        arr = solution.mergeSort(arr);
        System.out.println(Arrays.toString(arr));
    }
}

3. Quick Sort

public class QuickSort {
    public int[] quickSort(int[] array) {
        quickSort(array, 0, array.length - 1);
        return array;
    }

    public void quickSort(int[] array, int left, int right) {
        if(left >= right) return;
        Random r = new Random();
        int pivot_idx = left + r.nextInt(right - left + 1);
        int pivot = array[pivot_idx];
        swap(array, pivot_idx, right);
        int i = left, j = right - 1;
        while(i <= j) {
            if(array[i] < pivot) {
                i++;
            }
            else{
                swap(array, i, j--);
            }
        }
        swap(array, i, right);

        quickSort(array, left, i - 1);
        quickSort(array, i + 1, right);
    }

    private void swap(int[] array, int i, int j) {
        int tmp = array[i];
        array[i] = array[j];
        array[j] = tmp;
    }

    public static void main(String[] args) {
        int[] arr = {3,1,5,7,9,8,4,2,0};
        QuickSort solution = new QuickSort();
        arr = solution.quickSort(arr);
        System.out.println(Arrays.toString(arr));
    }
}

 

 

to be continued...