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IsValid.java

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package Algorithms.string;

import java.util.Stack;

public class IsValid {

public boolean isValid(String s) {

if (s == null || s.length() == 0) {

return true;

}

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

int len = s.length();

for (int i = 0; i < len; i++) {

char c = s.charAt(i);

if (stack.isEmpty()) {

// 栈为空的时候，不能放右括号

if (c == ']' || c == ')' || c == '}') {

return false;

}

stack.push(c);

continue;

}

// 栈不为空时，必须 要对应弹出相对应的括号

if (c == ')' && stack.peek() == '('

|| c == ']' && stack.peek() == '['

|| c == '}' && stack.peek() == '{'

) {

stack.pop();

// 或者继续压入左括号

} else if (c == '(' || c == '[' || c == '{') {

stack.push(c);

// 否则错误退出

} else {

return false;

}

}

return stack.isEmpty();

}

}