LeetCode:Expression Add Operators

题目链接：https://leetcode.com/problems/expression-add-operators/、

题目：

Given a string that contains only digits

0-9

and a target value, return all
possibilities to add binaryoperators (not unary)

+

,

-

,
or

*

between the digits so they evaluate to the target value.

Examples:

"123", 6 -> ["1+2+3", "1*2*3"] 
"232", 8 -> ["2*3+2", "2+3*2"]
"105", 5 -> ["1*0+5","10-5"]
"00", 0 -> ["0+0", "0-0", "0*0"]
"3456237490", 9191 -> []

题意：给定一个包含0-9的字符串和一个目标值，返回对字符串中字符的所有可能的运算来得到目标值

方法：递归

注意：在运算乘法时应该将前一步运算移除

public class Solution {
	public List<String> addOperators(String num, int target) {
		List<String> result = new ArrayList<String>();
		addOperatorsDFS(num, target, 0, 0, "", result);
		return result;
	}
	
	public void addOperatorsDFS(String num, int target, long diff, long curNum, String out, List<String> result) {
		if(num.length() == 0 && curNum == target) {
			result.add(out);
		}
		
		for(int i=1; i<=num.length(); i++) {
			String cur = num.substring(0,i);
			if(cur.length() > 1 && cur.charAt(0) == '0') 
				return;
			String next = num.substring(i);
					
			if(out.length() > 0) {
			    addOperatorsDFS(next, target, Long.parseLong(cur), curNum + Long.parseLong(cur), out+"+"+cur, result);
				addOperatorsDFS(next, target, -Long.parseLong(cur), curNum - Long.parseLong(cur), out+"-"+cur, result);
				addOperatorsDFS(next, target, diff*Long.parseLong(cur), curNum - diff + diff*Long.parseLong(cur), out+"*"+cur, result);
				
			} else {
				addOperatorsDFS(next, target, Long.parseLong(cur), Long.parseLong(cur), cur, result); 
			}
		}
	}
}