LeetCode -- Valid Parenthese

Question:

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.

Analysis:

问题描述：给出一个字符串，包含

'('

,

')'

,

'{'

,

'}'

,

'['

and

']'确定它是否是有效地匹配括号。

思路：一看到括号匹配问题肯定想到用栈。遇到左括号就进栈；遇到右括号若栈顶元素与之匹配则POP出栈顶元素，若不匹配则返回false。

注意特殊情况：如"{}[]()", 或者“{[}]”, 或者“{{}}{{”等情况。

Answer：

public class Solution {
public boolean isValid(String s) {
char[] ch = s.toCharArray();
int n = ch.length;
if(ch.length == 0 || ch.length % 2 != 0)
return false;
if(ch[0] == '}' || ch[0] == ']' || ch[0] == ')')
return false;
Stack<Character> st = new Stack<Character>();
st.add(ch[0]);
int i = 1;
while(!st.isEmpty() && i < n) {
if(ch[i] == '{' || ch[i] == '[' || ch[i] == '(') {
st.add(ch[i]);
i++;
} else {
if(st.isEmpty())
return false;
char c = st.pop();
if(c == '{' && ch[i] != '}' || c == '[' && ch[i] != ']'
|| c == '(' && ch[i] != ')')
return false;
i++;
if(i < n && (ch[i] == '{' || ch[i] == '[' || ch[i] == '(')) {
st.add(ch[i]);
i++;
}
}
}
System.out.println(i +" " +st.size());
if(!st.isEmpty() || i < n - 1)
return false;

return true;
}

}