Expression Add Operators

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一道求集合的dfs题。有很多的要注意的地方：

1. 为了处理，

"3456237490", 9191 -> []

我们得用long来处理数字，表现在：

long curNum = Long.valueOf(curStr);

2. i是小于等于num的size，

i <= num.length()

3. 对于乘法，要理解以下，来求解当前的结果

curRes - prevNum + prevNum * curNum

4. 对于第一个数字，直接将当前数字带入之后计算。

helper(list, nextNum, target, curStr, curNum, curNum);

public class Solution {public List<String> addOperators(String num, int target) {List<String> list = new LinkedList<>();helper(list, num, target, "", 0, 0);return list;}//private void helper(List<String> list, String num, int target, String temp, int curRes, int prevNum) {private void helper(List<String> list, String num, int target, String temp, long curRes, long prevNum) {if (curRes == target && num.length() == 0) {list.add(new String(temp));return ;}//for (int i = 1; i < num.length(); i++) {for (int i = 1; i <= num.length(); i++) {String curStr = num.substring(0, i);if (curStr.length() > 1 && curStr.charAt(0) == '0') {return;}//int curNum = Integer.valueOf(curStr);long curNum = Long.valueOf(curStr);String nextNum = num.substring(i);if (temp.length() != 0) {helper(list, nextNum, target, temp + '*' + curNum, curRes - prevNum + prevNum * curNum, prevNum * curNum);helper(list, nextNum, target, temp + '+' + curNum, curRes + curNum, curNum);helper(list, nextNum, target, temp + '-' + curNum, curRes - curNum, -curNum);} else {helper(list, nextNum, target, curStr, curNum, curNum);}}}
}

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