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The approach
Pass min and max bounds through recursion.
function isValidBST(root):
def validate(node, minVal, maxVal):
if not node:
return true
if node.val <= minVal or node.val >= maxVal:
return false
return validate(node.left, minVal, node.val) and
validate(node.right, node.val, maxVal)
return validate(root, float('-inf'), float('inf'))
time, space.
