20. Valid Parentheses

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Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

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Use if-else:

public class Solution {    public boolean isValid(String s) {        Stack<Character> st=new Stack<Character>();        int len=s.length();        for(int i=0;i<len;++i){            char temp=s.charAt(i);            if(temp=='('||temp=='['||temp=='{'){                st.push(temp);            }            else{                if(st.empty())                    return false;                char ch=st.pop();                if(temp==')'&&ch!='(')                    return false;                else if(temp==']'&&ch!='[')                    return false;                else if(temp=='}'&&ch!='{')                    return false;                 }             }        return st.empty();    }}

use switch:

public class Solution {    public boolean isValid(String s) {        Stack<Character> st=new Stack();        for(int i=0;i<s.length();++i){            switch(s.charAt(i)){                case('('):                case('{'):                case('['):{                    st.push(s.charAt(i));                    break;                }                case(')'):{                    if(st.empty()||st.pop()!='(')                        return false;                    break;                    }                case(']'):{                    if(st.empty()||st.pop()!='[')                        return false;                    break;                    }                case('}'):{                    if(st.empty()||st.pop()!='{')                        return false;                    break;                    }                default:break;                }            }        return st.empty();                }}

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