归并排序

Posted 2020-12-17 faded828x

归并排序：

先分治后归并。

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/* Java program for Merge Sort */
class MergeSort { 
    // Merges two subarrays of arr[]. 
    // First subarray is arr[l..m] 
    // Second subarray is arr[m+1..r] 
    void merge(int arr[], int l, int m, int r) 
    { 
        // Find sizes of two subarrays to be merged 
        int n1 = m - l + 1; 
        int n2 = r - m; 
  
        /* Create temp arrays */

　　　　　　int L[] = Arrays.copyOfRange(arr, l, m + 1); 

        　int R[] = Arrays.copyOfRange(arr, m+1, r+1); 

　　　　/* Merge the temp arrays */
  
        // Initial indexes of first and second subarrays 
        int i = 0, j = 0; 
  
        // Initial index of merged subarry array 
        int k = l; 
        while (i < n1 && j < n2) { 
            if (L[i] <= R[j]) { 
                arr[k] = L[i]; 
                i++; 
            } 
            else { 
                arr[k] = R[j]; 
                j++; 
            } 
            k++; 
        } 
  
        /* Copy remaining elements of L[] if any */
        while (i < n1) { 
            arr[k] = L[i]; 
            i++; 
            k++; 
        } 
  
        /* Copy remaining elements of R[] if any */
        while (j < n2) { 
            arr[k] = R[j]; 
            j++; 
            k++; 
        } 
    } 
  
    // Main function that sorts arr[l..r] using 
    // merge() 
    void sort(int arr[], int l, int r) 
    { 
        if (l < r) { 
            // Find the middle point 
            int m = (l + r) / 2; 
  
            // Sort first and second halves 
            sort(arr, l, m); 
            sort(arr, m + 1, r); 
  
            // Merge the sorted halves 
            merge(arr, l, m, r); 
        } 
    }