below are my codes for Leetcode 20. (Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

Open brackets must be closed by the same type of brackets. Open brackets must be closed in the correct order.)

When the input is "(])" I still got true. Can anyone let me know what is wrong with my code? Thanks!

class Solution {
    public boolean isValid(String s) {
        
        Stack<Character> stack = new Stack<>();
        for(char c: s.toCharArray()){
            if(c == '(' || c == '[' || c == '{'){
                stack.push(c);
            }else{
                if(stack.empty()){
                    return false;
                }
                if(c == ')' && stack.peek() == '('){
                    stack.pop();
                }
                if(c == ']' && stack.peek() == '['){
                    stack.pop();
                }
                if(c == '}' && stack.peek() == '{'){
                    stack.pop();
                }
            }
            
        }return stack.empty();
    }
}

CodePudding user response：

On the second iteration of the for loop you have char ], it doesn't match the first conditional so it goes on to the else block. None of the other if statements match, therefor it doesn't know what to do and just starts on the 3rd iteration of the loop, where it sees ) and also sees ( on peek so returns empty. This is where the issue lies. You'll need to add an additional else inside your else block to catch anything that doest match the 4 ifs.

In order to fix this particular test, add a check for the ] character only. if you see that character and you havent seen any [s then return false

Hopefully that helps, if not, let me know and I can try to clarify more.