22.括号生成
比较容易想到暴力解法，生成所有的括号然后检验是否有效。
public List<String> generateParenthesis(int n) {
List<String> combinations = new ArrayList<String>();
generateAll(new char[2 * n], 0, combinations);
return combinations;
}
public void generateAll(char[] current, int pos, List<String> result) {
if (pos == current.length) {
if (valid(current)) {
result.add(new String(current));
}
} else {
current[pos] = '(';
generateAll(current, pos + 1, result);
current[pos] = ')';
generateAll(current, pos + 1, result);
}
}
public boolean valid(char[] current) {
int balance = 0;
for (char c: current) {
if (c == '(') {
++balance;
} else {
--balance;
}
if (balance < 0) {
return false;
}
}
return balance == 0;
}回溯法--只生成有效的括号，
public List<String> generateParenthesis(int n) {
List<String> ans = new ArrayList<>();
backtrack(ans,new StringBuilder(),0,0,n);
return ans;
}
public void backtrack(List<String> ans, StringBuilder cur, int open, int close, int max) {
if (cur.length() == max*2) {
ans.add(cur.toString());
return;
}
if (open < max) {
cur.append('(');
backtrack(ans,cur,open+1,close,max);
cur.deleteCharAt(cur.length()-1);
}
if (close < open) {
cur.append(')');
backtrack(ans, cur, open, close+1,max);
cur.deleteCharAt(cur.length()-1)
}
