以一个枢纽值二分一个数组

划分算法由两个指针开始，分别指向数组的两头。在左边的指针向右移动，右边的指针向左移动。左边的指针leftPtr初始化为第一个数据项，右边的指针rightPtr初始化为数组的最后一项。算法如下：

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import java.util.Random;

public class Partition {
    private long[] theArr;
    private int nElems;

    public Partition(int max) {
        theArr = new long[max];
        nElems = 0;
    }

    public void insert(long value) {
        theArr[nElems++] = value;
    }

    public int size() {
        return nElems;
    }

    public void display() {
        System.out.print("A = ");
        for (int j = 0; j < nElems; j++) {
            System.out.print(theArr[j] + " ");
        }
        System.out.println();
    }

    public int partitionIt(int left, int right, long pivot) {
        int leftPtr = left - 1;
        int rightPtr = right + 1;
        while (true) {
            // keep moving to right until current value is larger than pivot
            while (leftPtr < right && theArr[++leftPtr] < pivot) {
                //
            }

            // keep moving to left until current value is smaller than pivot
            while (rightPtr > left && theArr[--rightPtr] > pivot) {
                //
            }

            // if left pointer meet the right pointer, break
            if (leftPtr >= rightPtr) {
                break;
            } else {
                swap(leftPtr, rightPtr);
            }
        }

        // return pivot position
        return leftPtr;
    }

    public void swap(int dex1, int dex2) {
        long temp = theArr[dex1];
        theArr[dex1] = theArr[dex2];
        theArr[dex2] = temp;
    }
}

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end.