快排和堆排序

別看這個簡單也基礎，但是真的面試的時候會讓你寫，紙上手寫，嗯

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快排

private static void quickSort(int[] test, int start, int end) {
        //quick sort的主程序
        if(start < end){
            int q = partition(test, start, end);
            quickSort(test, start, q-1);
            quickSort(test, q+1, end);
        }

    }

    private static int partition(int[] test, int start, int end) {
        //完成一次劃分
        //主元選擇test[end]
        int pivot = test[end];
        int i = start - 1;
        for(int j = start; j < end; j++){
            if(test[j] <= pivot){
                i++;
                //交換test[i]和test[j]
                int t = test[i];
                test[i] = test[j];
                test[j] = t;
            }
        }
        //i+1的位置即是pivot的位置
        //交換test[i+1]和pivot
        int t1 = test[i+1];
        test[i+1] = test[end];
        test[end] = t1;
        return i+1;
    }

堆排序

public static void heapSort(int[] test){
         //建堆
         buildHeap(test);
         for(int heapSize = test.length - 1; heapSize >= 1; heapSize--){
             //交換test[heapSize]和test[0]的位置
             int t = test[0];
             test[0] = test[heapSize];
             test[heapSize] = t;
             //維護堆的性質
             maxHeapify(test, heapSize, 0);
         }
     }
    private static void maxHeapify(int[] test, int heapSize, int i) {
        //維護堆的性質
        int left = 2 * i + 1;
        int right = 2 * i + 2;
        //找出三者最大的
        int largest;
        if(left < heapSize && test[left] > test[i]){
            largest = left;
        }else{
            largest = i;
        }
        if(right < heapSize && test[right] > test[largest]){
            largest = right;
        }
        if(i != largest){
            //交換test[i]和test[largest]
            int t = test[i];
            test[i] = test[largest];
            test[largest] = t;
            //遞歸
            maxHeapify(test, heapSize, largest);
        }

    }
    private static void buildHeap(int[] test) {
        //建堆
        for(int i = test.length / 2; i >= 0; i--){
            maxHeapify(test, test.length, i);
        }

    }

main函數:

int test = {2,3,1,1,5,6,7,7,8,4,9,11};
//quickSort(test, 0, test.length-1);
heapSort(test);
for(int x : test){
    System.out.print(x+" ");
}