面试题： generate an equation, by inserting operator add ("+") and minus ("-") among the array to make equationExpression == 0

package com.Amazon.interview;



/**

 * @Author: weblee

 * @Email: [email protected]

 * @Blog: http://www.cnblogs.com/lkzf/

 * @Time: 2014年10月25日下午5:14:33

 * 

 *************        function description ***************

 * 

 *        Question:

 * 

 *        Given an array with positive integers and another integer for

 *        example{7 2 4} 9, you are required to generate an equation, by

 *        inserting operator add ("+") and minus ("-") among the array . The

 *        left side of equation are consist of the array and the right side of

 *        equation is the integer. here the result is 7-2+4=9

 * 

 * 

 * 

 *        Rules:

 * 

 *        Don't include any space in the generated equation. In case there is no

 *        way to create the equation, please output "Invalid". For example {1 1}

 *        10, output is "Invalid"

 * 

 *        There is no operator "+" or "-" in front of the first number: Don't

 *        change the order of the numbers. For example: {7 2 4} 9. 7-2+4=9 is

 *        correct answer, 4-2+7=9 is wrong answer. There could be multiple

 *        input, meaning your function could be called multiple times. Do

 *        remember print a new line after the call.

 * 

 *        The length of the integer array is from 1 to 15( include 1 and 15). If

 *        the length is 1, for example the input {7} 7, the output is 7=7

 * 

 *        Sample Input and Output:

 * 

 *        Input:

 * 

 *        1 2 3 4 10

 * 

 *        1 2 3 4 5

 * 

 *        Output:

 * 

 *        1+2+3+4=10

 * 

 *        Invalid

 * 

 *

 * 

 ****************************************************

 */



public class GenerateEquation {

    public static void createEqualAndPrint(int[] a, int n, int target) {

    if (a == null || a.length == 0 || a.length != n || n == 0) {

        System.out.println("Invalid");



        return;

    }



    if (n < 1 || n > 15) {

        System.out.println("Invalid");

        return;

    }



    int i = n - 1;

    int v = 1;

    while (i > 1) {

        v = (v << 1) + 1;

        i--;

    }



    int sum = 0;

    String s = null;

    while (v > 0) {

        sum = a[0];

        s = a[0] + "";

        for (int j = n - 1; j > 0; j--) {

        int c = v >> (j - 1);

        if ((c & 1) == 1) {

            sum += a[n - j];

            s += "+" + a[n - j];

        } else {

            sum -= a[n - j];

            s += "-" + a[n - j];

        }

        }

        if (sum == target) {

        System.out.println(s + "=" + target);

        return;

        } else {

        v--;

        }

    }



    System.out.println("Invalid.");

    }



    /**

     * @param args

     */

    public static void main(String[] args) {

    int[] a = { 1, 2, 3, 4 };



    createEqualAndPrint(a, 4, 2);

    }



}