Merge k Sorted Arrays

Given k sorted integer arrays, merge them into one sorted array.

Challenge

Do it in O(N log k).

• N is the total number of integers.
• k is the number of arrays.

思路： 1.合并成1个数组，调用快速排序。  时间复杂度 O(NlogN + N)

　　　 2.二分法 + merge 时间复杂度O（Nlogk）。

public class Solution {
    /**
     * @param arrays k sorted integer arrays
     * @return a sorted array
     */
    public List<Integer> mergekSortedArrays(int[][] arrays) {
        List<Integer> list = new ArrayList<>();
        if (arrays == null || arrays.length == 0) {
            return list;
        }
        int[] tmp = mergeHelper(arrays, 0 , arrays.length - 1);
        for (int i = 0; i < tmp.length; i++) {
            list.add(tmp[i]);
        }
        return list;
    }
    
    private int[] mergeHelper(int[][] arrays, int begin, int end) {
        if (begin >= end) {
            return arrays[begin];
        }
        int mid = begin + (end - begin) / 2;
        int[] a = mergeHelper(arrays, begin, mid);
        int[] b = mergeHelper(arrays, mid + 1, end);
        return merge(a, b);
    }
    
    private int[] merge(int[] a, int[] b) {
        int len = a.length + b.length;
        int[] result = new int[len];
        int i = 0, j = 0, k = 0;
        for (k = 0; k < len; k++) {
            if (i < a.length && (j == b.length || a[i] < b[j])) {
                result[k] = a[i];
                i++;
            } else {
                result[k] = b[j];
                j++;
            }
        }
        return result;
        
    }
}