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: likaiweb@163.com
* @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);
}

}