LeetCode 20 -- 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.

主要思想还是盏。

public static boolean isValid(String s) {
HashMap<Character, Character> map = new HashMap<Character, Character>();
map.put('(', ')');
map.put('[', ']');
map.put('{', '}');

Stack<Character> stack = new Stack<Character>();

for (int i = 0; i < s.length(); i++) {
char curr = s.charAt(i);

if (map.keySet().contains(curr)) {
stack.push(curr);
} else if (map.values().contains(curr)) {
if (!stack.empty() && map.get(stack.peek()) == curr) {
stack.pop();
} else {
return false;
}
}
}

return stack.empty();
}

public class S20 {

public static void main(String[] args) {

}

// 用stack来检查
public boolean isValid(String s) {
Stack<Character> stack = new Stack<Character>();
for(int i=0; i<s.length(); i++){
char c = s.charAt(i);
// 如果遇到前括号就压入栈
if(c=='(' || c=='[' || c=='{'){
stack.push(c);
}else if(c==')' || c==']' || c=='}'){       // 遇到后括号就出栈
if(stack.size() == 0){  //  说明后括号太多了
return false;
}
char cpop = stack.pop();
if(cpop=='(' && c==')'){
continue;
}else if(cpop=='[' && c==']'){
continue;
}else if(cpop=='{' && c=='}'){
continue;
}
return false;
}
}
return stack.size()==0;
}

}