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IsValidBST.java

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package Algorithms.tree;

/**

* Definition for binary tree

* public class TreeNode {

* int val;

* TreeNode left;

* TreeNode right;

* TreeNode(int x) { val = x; }

* }

*/

public class IsValidBST {

public boolean isValidBST(TreeNode root) {

return isValidBSTHelp(root).isValidBST;

}

public ReturnType isValidBSTHelp(TreeNode root) {

ReturnType ret = new ReturnType(Integer.MAX_VALUE, Integer.MIN_VALUE, true);

if (root == null) {

return ret;

}

ReturnType left = isValidBSTHelp(root.left);

ReturnType right = isValidBSTHelp(root.right);

/* the left tree and the right tree should both be Valid BST.

And the value of the root should be in the middle.

*/

if (!left.isValidBST

|| !right.isValidBST

|| left.max >= root.val

|| right.min <= root.val

) {

ret.isValidBST = false;

return ret;

}

// get the min value of the tree;

ret.min = Math.min(left.min, root.val);

// get the max value of the tree, consider the right node may be null;

ret.max = Math.max(right.max, root.val);

return ret;

}

public class ReturnType {

int min;

int max;

boolean isValidBST;

ReturnType(int min, int max, boolean isValidBST) {

this.min = min;

this.max = max;

this.isValidBST = isValidBST;

}

}

}