Algorithms
Assign binary operators to evaluate to target value
Given a string that contains only digits 0-9 and a target value, return all possibilities to add binary operators (not unary) +, -, or * between the digits so they evaluate to the target value.
For example:
“232”, 8 -> [“2*3+2”, “2+3*2”]
“00”, 0 -> [“0+0+0”, “0-0”, “0*0”]
“102”, 2 -> [“1*0+2”]
“45435”, 9191 -> []
public List<String> addOperators(String num, int target) {
List<String> result = new ArrayList<String>();
if(num == null || num.length() == 0) return result;
addOperator(num, 0, "", target, 0, 0, result);
return result;
}
private void addOperator(String num, int curPos, String curPath, int target, long curEval, long prevOpnd, List<String> res){
if(curPos == num.length()){
if(curEval == target){
res.add(curPath);
}
return;
}
for(int i = curPos; i<num.length(); i++){
if(i != curPos && num.charAt(curPos) == '0'){
break;
}
long curOprnd = Long.parseLong(num.substring(curPos, i+1));
if(curPos == 0){
addOperator(num, i+1, curPath+curOprnd, target, curOprnd, curOprnd, res);
}
else{
addOperator(num, i+1, curPath+"+"+curOprnd, target, curEval+curOprnd, curOprnd, res);
addOperator(num, i+1, curPath+"-"+curOprnd, target, curEval-curOprnd, -curOprnd, res);
addOperator(num, i+1, curPath+"*"+curOprnd, target, curEval-prevOpnd+prevOpnd*curOprnd, prevOpnd*curOprnd, res);
}
}
}