LeetCode Top 100 Liked Questions 22. Generate Parentheses (Java版； Medium)

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LeetCode Top 100 Liked Questions 22. Generate Parentheses (Java版; Medium)

题目描述

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.For example, given n = 3, a solution set is:["((()))","(()())","(())()","()(())","()()()"
]

class Solution {public List<String> generateParenthesis(int n) {List<String> list = new ArrayList<>();if(n==0){return list;}StringBuilder sb = new StringBuilder();core(list, sb, 0, 0, n);return list;}private void core(List<String> list , StringBuilder sb, int L, int R, int n){//if(L==n){int count = 0;while(R<n){sb.append(')');R++;count++;}list.add(sb.toString());for(int i=0; i<count; i++){sb.deleteCharAt(sb.length()-1);}return;}if(L<n){sb.append('(');core(list, sb, L+1, R, n);sb.deleteCharAt(sb.length()-1);}if(R<L){sb.append(')');core(list, sb, L, R+1, n);sb.deleteCharAt(sb.length()-1);}}
}

第一次做, 有点混乱, 想着采用改变现场恢复现场的常规手段做, 结果绕进去了; 混乱的原因在于要控制左括号和右括号的数量, 一般只控制一种字符的数量, 这次控制了俩, 所以乱了, 不过这两种字符的数量是有不等式约束的: 左括号的数量小于等于n, 右括号的熟练小于等于左括号, 做题的时候没有利用这一点; 创建两个变量分别记录左右括号的数量

创建两个变量记录左右括号出现的次数
使用不等式进行约束

/*
原则:
左括号数量小于等于n
右括号数量小于等于左括号数量
*/
import java.util.ArrayList;
import java.util.Stack;class Solution {public List<String> generateParenthesis(int n) {List<String> res = new ArrayList<>();if(n<1)return res;Stack<String> s = new Stack<>();//s.push("(");StringBuilder sb = new StringBuilder();Core(res, "", 0, 0, n);return res;}public void Core(List<String> res, String curr, int open, int close, int n){//base caseif(open==n){//最后要保证左右括号数量相等while(close<open){curr += ")";close++;}res.add(curr);return;}/*另一种base case, 从curr的长度出发if(curr.length()==2*n)res.add(curr);*///只要open小于n, 就可以加(if(open<n){Core(res, curr+"(", open+1, close, n);}//只要close<open, 就可以加)if(close<open){Core(res, curr+")", open, close+1, n);}}
}

题解

class Solution {public List<String> generateParenthesis(int n) {List<String> ans = new ArrayList();backtrack(ans, "", 0, 0, n);return ans;}public void backtrack(List<String> ans, String cur, int open, int close, int max){if (cur.length() == max * 2) {ans.add(cur);return;}if (open < max)backtrack(ans, cur+"(", open+1, close, max);if (close < open)backtrack(ans, cur+")", open, close+1, max);}
}

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LeetCode Top 100 Liked Questions 22. Generate Parentheses (Java版； Medium)

welcome to my blog

LeetCode Top 100 Liked Questions 22. Generate Parentheses (Java版; Medium)

题目描述

题解

LeetCode Top 100 Liked Questions 22. Generate Parentheses (Java版； Medium)相关推荐

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