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MergeSort.java
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92 lines (74 loc) · 2.24 KB
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package com.algorithms;
/**
* 经典问题: 归并排序
* Created by xifeng.yang on 2020/2/2
*/
public class MergeSort {
public static void main(String[] args) {
MergeSort o = new MergeSort();
String[] a = {"M", "E", "R", "G", "E", "S", "O", "R", "T", "E", "X", "A", "M", "P", "L", "E", "H"};
o.sortBottomUp(a);
show(a);
}
/**
* 自顶向下的归并排序
*
* @param a
*/
public void sort(Comparable[] a) {
Comparable[] aux = new Comparable[a.length];
sort(a, aux, 0, a.length - 1);
}
/**
* 自底向上的归并排序
*
* @param a
*/
public void sortBottomUp(Comparable[] a) {
int len = a.length;
Comparable[] aux = new Comparable[len];
for (int step = 1; step < len; step *= 2) {
//从lo起所剩元素>step时才需要进行merge.
for (int lo = 0; lo < len - step; lo += step * 2) {
merge(a, aux, lo, lo + step - 1, Math.min(lo + step + step - 1, len - 1));
}
}
}
// merge-sort a[lo..hi] using auxiliary array aux[lo..hi]
private void sort(Comparable[] a, Comparable[] aux, int lo, int hi) {
if (lo >= hi) {
return;
}
int mid = lo + (hi - lo) / 2;
sort(a, aux, lo, mid);
sort(a, aux, mid + 1, hi);
merge(a, aux, lo, mid, hi);
}
private void merge(Comparable[] a, Comparable[] aux, int lo, int mid, int hi) {
for (int k = lo; k <= hi; k++) {
aux[k] = a[k];
}
System.arraycopy(a, lo, aux, lo, hi + 1 - lo);
// merge back to a[]
int i = lo, j = mid + 1;
for (int k = lo; k <= hi; k++) {
if (i > mid) {
a[k] = aux[j++];
} else if (j > hi) {
a[k] = aux[i++];
} else if (less(aux[j], aux[i])) {
a[k] = aux[j++];
} else {
a[k] = aux[i++];
}
}
}
private static boolean less(Comparable v, Comparable w) {
return v.compareTo(w) < 0;
}
private static void show(Comparable[] a) {
for (int i = 0; i < a.length; i++) {
System.out.println(a[i]);
}
}
}