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public class Solution {
    public boolean isValidSerialization(String preorder) {
        int i = traverse(preorder, 0);
        return i >= preorder.length();
        
        
    }
    
    public static int traverse(String preorder, int i) {
        if (i >= preorder.length()) return i;
        if (i < 0) return i;        
        while (preorder.charAt(i) == ',') {
            i++;
        }
    
        char root = preorder.charAt(i);

        if (root == '#') return i + 1;
        
        while (i < preorder.length() && preorder.charAt(i) != ',') {
        	i++;
        }
        
        int left = traverse(preorder, i);
        if (left >= preorder.length()) return -i;
        int right = traverse(preorder, left);
        return right;
        
    }	
}

public boolean isValidSerialization(String preorder) {
    String[] nodes = preorder.split(",");
    int diff = 1;
    for (String node: nodes) {
        if (--diff < 0) return false;
        if (!node.equals("#")) diff += 2;
    }
    return diff == 0;
}

public class Solution {
    public boolean isValidSerialization(String preorder) {
        Stack<String> stack = new Stack<>();
        String[] strs = preorder.split(",");
        for (int i = 0; i < strs.length; i++) {
            while (strs[i].equals("#") && !stack.isEmpty() && stack.peek().equals("#")) {
                    stack.pop();
                    if (stack.isEmpty()) return false;
                    stack.pop();
            } 
            stack.push(strs[i]);
        }
        return stack.size() == 1 && stack.peek().equals("#");
    }
}