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DifferentWaysToAddParentheses.java
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DifferentWaysToAddParentheses.java
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import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
/**
* 241. Different Ways to Add Parentheses
* 分治
* @author LBW
*/
public class DifferentWaysToAddParentheses {
public List<Integer> diffWaysToCompute(String expression) {
int len = expression.length();
int idx = 0, sign = 1, number = 0;
if (expression.charAt(idx) == '-') {
sign = -1;
idx += 1;
}
while (idx < len && !isOperator(expression.charAt(idx))) {
number = number * 10 + expression.charAt(idx) - '0';
idx += 1;
}
if (idx == len) {
return Collections.singletonList(sign * number);
}
// divide and conquer
List<Integer> res = new ArrayList<>();
for (int i = 0; i < len; i++) {
if (isOperator(expression.charAt(i))) {
List<Integer> left = diffWaysToCompute(expression.substring(0, i));
List<Integer> right = diffWaysToCompute(expression.substring(i + 1));
for (int l: left) {
for (int r: right) {
switch (expression.charAt(i)) {
case '+':
res.add(l + r);
break;
case '-':
res.add(l - r);
break;
case '*':
res.add(l * r);
break;
}
}
}
}
}
return res;
}
private boolean isOperator(char c) {
return c == '+' || c == '-' || c == '*';
}
}