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43 lines
793 B
43 lines
793 B
package class30;
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public class Problem_0098_ValidateBinarySearchTree {
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public static class TreeNode {
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int val;
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TreeNode left;
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TreeNode right;
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}
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public boolean isValidBST(TreeNode root) {
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if (root == null) {
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return true;
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}
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TreeNode cur = root;
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TreeNode mostRight = null;
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Integer pre = null;
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boolean ans = true;
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while (cur != null) {
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mostRight = cur.left;
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if (mostRight != null) {
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while (mostRight.right != null && mostRight.right != cur) {
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mostRight = mostRight.right;
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}
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if (mostRight.right == null) {
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mostRight.right = cur;
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cur = cur.left;
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continue;
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} else {
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mostRight.right = null;
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}
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}
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if (pre != null && pre >= cur.val) {
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ans = false;
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}
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pre = cur.val;
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cur = cur.right;
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}
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return ans;
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}
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}
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