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最小的k个数

admin 11月 13, 2020 0

给定一个无序数组，找到其中最小的K个数

利用最大堆,只要维护堆内有k个数即可

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public static int[] getMinKNumByHeap(int[]arr ,int k){
    if(arr==null ||arr.length<k|| k<0){
        return null;
    }
    int[] kHeap = new int[k];
    for(int i =0 ; i<k; i++){
        heapInsert(kHeap,arr[i],i);
    }
    for(int i=k; i<arr.length; i++){
        if(kHeap[0]>arr[i]){
            kHeap[0] = arr[i];
            heapify(kHeap, 0, k);
        }
    }
    return kHeap;
} 
//建堆的过程
public static void (int[] heap, int value, int index){
    heap[index] = value;
    int parent = (index-1)/2;
    while(index!=0){
        if(heap[index]>heap[parent]){
            swap(heap,index,parent);
            index = parent;
        }else{
            break;
        }
    }
}
//调整的过程
public static void heapify(int[] heap, int index, int heapSize){
    int left = 2*index +1;
    int right = 2*index +2;
    int largest = index;
    while(left < heapSize){
        if(heap[index]<heap[left]){
            //swap(heap,index,left);
            largest = left;
        }
        if(right < heapSize && heap[largest]<heap[right]){
            largest = right;
        }
        if(index!=largest){
            swap(heap,index,largest);
        }else{
            break;
        }
        index = largest;
        left = 2*index +1;
        right = 2*index +2;
    }

}
public static void swap(int[] arr, int low, int high){
    int temp = arr[low];
    arr[low] = arr[high];
    arr[high] = temp;
}

快排思想, 无需全部排序,保证前k个数有序即可

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public static int partition(int[] arr,int start,int end){
    if(arr==null || arr.length==0){
        return -1; 
    }
    int l = start;
    int r = end;
    int temp = arr[l];
    if(l<r){
        while(l<r && temp >= arr[r]){
            r--;
        }
        if(l<r){
            arr[l++]=arr[r];
        }
        while(l<r && temp < arr[l]){
            l++;
        }
        if(l<r){
            arr[r--] = arr[l];
        }
    }
    arr[l] = temp;
    return l;
}
//利用快排思想
public static void getKNum(int[] arr,int start,int end, int k){
    int index = partition(arr, start, end);
    while(index != k-1){
        if(index < k-1){
            index = partition(arr, index+1, end);
        }else if(index > k-1){
            index = partition(arr, start, index-1);
        }else{
            return;
        }
    }
}
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