98.Validate Binary Search Tree

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Solution 1: accepted

In-order traversal.

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class Solution {
    private int last = Integer.MIN_VALUE;
    private boolean first = true; // for special case [-2147483648]
    public boolean isValidBST(TreeNode root) {
        if (root == null) {
            return true;
        }
        
        if (!isValidBST(root.left)) {
            return false;
        }
        
        if (!first && last >= root.val) {
            return false;
        }
        first = false;
        last = root.val;
        
        if (!isValidBST(root.right)) {
            return false;
        }
                
        return true;
    }
}