AC JAVA solution

  • 2

    The idea is the max num of the left subtree must smaller than root in the mean while the min num of the right subtree must bigger than root.

    public class Solution {
        public boolean isValidBST(TreeNode root) {
            if(root == null)    return true;
            boolean left = (root.left == null)? true:(getMax(root.left)<root.val)?isValidBST(root.left):false;
            boolean right = (root.right == null)? true:(getMin(root.right)>root.val)?isValidBST(root.right):false;
            return left && right;
        private int getMin(TreeNode node){
            while(node.left != null)
                node = node.left;
            return node.val;
        private int getMax(TreeNode node){
            while(node.right != null)
                node = node.right;
            return node.val;

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