301. Remove Invalid Parentheses

Remove the minimum number of invalid parentheses in order to make the input string valid. Return all possible results.

Note: The input string may contain letters other than the parentheses ( and ).

Example 1:

Input: "()())()"
Output: ["()()()", "(())()"]

Example 2:

Input: "(a)())()"
Output: ["(a)()()", "(a())()"]

Example 3:

Input: ")("
Output: [""]

 

Approach #1: C++.

class Solution {
public:
    vector<string> removeInvalidParentheses(string s) {
        int l = 0;
        int r = 0;
     // calculate the mismatch parentheses. for (char c : s) { l += (c == '('); if (l == 0) { r += (c == ')'); } else { l -= (c == ')'); } } vector<string> ans; dfs(s, 0, l, r, ans); return ans; } private: bool isValid(string& s) { int count = 0; for (char c : s) { if (c == '(') count++; if (c == ')') count--; if (count < 0) return false; } return count == 0; } void dfs(string s, int start, int l, int r, vector<string>& ans) { if (l == 0 && r == 0) { if (isValid(s))  // judge the string is valid. ans.push_back(s); return; } for (int i = start; i < s.length(); ++i) { if (i != start && s[i] == s[i-1]) continue;
        // s[i] is '(' or ')' if (s[i] == '(' || s[i] == ')') { string curr = s; curr.erase(i, 1); if (r > 0) dfs(curr, i, l, r-1, ans); else if (l > 0) dfs(curr, i, l-1, r, ans); } } } };

  

Analysis:

step1: compute min number of '(' and ')' to remove, unbalanced ')' + unblanced '('

step2: try all possible ways to remove r '(' and l ')'. Remove '(' first to make prefix valid.

step3: when r == 0 and l == 0, judging the string is or not fulfiled conditions.

 

posted @ 2018-12-10 21:58  Veritas_des_Liberty  阅读(207)  评论(0编辑  收藏  举报