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| class Solution {
public List<String> generateParenthesis(int n) {
List<String> ans = new ArrayList<>();
helper(ans, new StringBuilder(), n, n);
return ans;
}
private void helper(List<String> ans, StringBuilder currString, int openLeft, int closedLeft) {
if (openLeft == 0 && closedLeft == 0) {
ans.add(currString.toString());
return;
}
if (openLeft > 0) {
currString.append('(');
helper(ans, currString, openLeft - 1, closedLeft);
currString.deleteCharAt(currString.length() - 1);
}
if (closedLeft > openLeft) {
currString.append(')');
helper(ans, currString, openLeft, closedLeft - 1);
currString.deleteCharAt(currString.length() - 1);
}
}
}
|
经典的permutation类型的递归. 给一个不同递归函数共有的东西加东西再减东西.
时间复杂度太复杂了.
空间复杂度也是.
