
Valid Parentheses
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.
思路:题目整体上比較明白,是对括号是否有效做推断,算法上是用栈来实现,单边入栈。配对之后出栈,最后推断栈是否为空。不为空说明最后没有配对完毕。返回false.
详细代码例如以下:
public class Solution {
public boolean isValid(String s) {
Stack<Character> st = new Stack<Character>();
char[] ch = s.toCharArray();
int len = ch.length;
//假设长度为奇数,肯定无效
if((len & 1) != 0){
return false;
} for(int i = 0; i < len; i++){
if(st.isEmpty()){//栈为空,直接入栈
st.push(ch[i]);
}else{//不为空则讨论栈顶元素是否与ch[i]配对。 配对也出栈
if(isMatch(st.peek(),ch[i])){//是否配对
st.pop();//栈顶元素出栈
}else{
st.push(ch[i]);//不配对入栈
}
}
}
return st.isEmpty();
}
//a为栈顶元素,b为字符串最前元素
public static boolean isMatch(char a, char b){
switch(a){
case '(': if(b == ')') return true; else return false;
case '{': if(b == '}') return true; else return false;
case '[': if(b == ']') return true; else return false;
default : return false;
}
}
}