Western Tech Interview Prep Session #2 Answers

reverseBetween.java

    public ListNode reverseBetween(ListNode head, int m, int n) {
    
    if(m==n) {
        return head;
    }
    
    ListNode curr = head;
    ListNode tempM = null; 
    ListNode tempAfterN = null; 
    ListNode tempBeforeM = null; 
    
    int nodeCount = 1 ;
    
    while (curr!=null && nodeCount<=n){
    
        if (nodeCount == m-1){ 
            tempBeforeM = curr; 
        } 
        
        if (nodeCount == m) { 
            tempM = curr; 
        } 
        
        if (nodeCount == n) { 
            tempAfterN = curr.next; 
            curr.next = null; 
        } 
        
        ++nodeCount; 
        curr = curr.next;
    }
    
    if (tempM.next == null) {
        return head;
    }
    
    ListNode newTemp = tempM; 
    ListNode nextNode = newTemp.next; 
    newTemp.next = tempAfterN;
    
    while (newTemp!=null && nextNode!=null){ 
        ListNode tempNode = nextNode.next; 
        nextNode.next = newTemp; 
        newTemp=nextNode; 
        nextNode=tempNode; 
    }
    
    if (tempBeforeM !=null ) {
        tempBeforeM.next = newTemp; 
    } else { 
        return newTemp;
    }
    
    return head;
    
    } 

reverseList.java

    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        
        ListNode curr = head;
        ListNode nextNode = head.next;
        head.next = null;
    
        while(curr!=null&& nextNode!=null){
            ListNode temp = nextNode.next;
            nextNode.next = curr;
            curr = nextNode;
            nextNode = temp;
        }
 
    return curr;

    }

validParentheses.java

    public boolean isValid(String s) {
      HashMap<Character, Character> pareMap = new HashMap<Character, Character>();
    	pareMap.put('(', ')');
    	pareMap.put('[', ']');
    	pareMap.put('{', '}');
     
    	Stack<Character> charStack = new Stack<Character>();
     
    	for (int i = 0; i < s.length(); i++) {
    		char currChar = s.charAt(i);
     
    		if (pareMap.containsKey(currChar)) {
    			charStack.push(currChar);
    		} else if (pareMap.containsValue(currChar)) {
    			if (!charStack.empty() && pareMap.get(charStack.peek()) == currChar) {
    				charStack.pop();
    			} else {
    				return false;
    			}
    		}
    	}
     
    	return charStack.empty();
    }