235. Lowest Common Ancestor of a Binary Search Tree
Easy
Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.
According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”
Given binary search tree: root = [6,2,8,0,4,7,9,null,null,3,5]
Example 1:
Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8
Output: 6
Explanation: The LCA of nodes2and8is6.
Example 2:
Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
Output: 2
Explanation: The LCA of nodes2and4is2, since a node can be a descendant of itself according to the LCA definition.
Note:
- All of the nodes' values will be unique.
- p and q are different and both values will exist in the BST.
package leetcode.easy; /**
* Definition for a binary tree node. public class TreeNode { int val; TreeNode
* left; TreeNode right; TreeNode(int x) { val = x; } }
*/
public class LowestCommonAncestorOfABinarySearchTree {
public TreeNode lowestCommonAncestor1(TreeNode root, TreeNode p, TreeNode q) { // Value of current node or parent node.
int parentVal = root.val; // Value of p
int pVal = p.val; // Value of q;
int qVal = q.val; if (pVal > parentVal && qVal > parentVal) {
// If both p and q are greater than parent
return lowestCommonAncestor1(root.right, p, q);
} else if (pVal < parentVal && qVal < parentVal) {
// If both p and q are lesser than parent
return lowestCommonAncestor1(root.left, p, q);
} else {
// We have found the split point, i.e. the LCA node.
return root;
}
} public TreeNode lowestCommonAncestor2(TreeNode root, TreeNode p, TreeNode q) { // Value of p
int pVal = p.val; // Value of q;
int qVal = q.val; // Start from the root node of the tree
TreeNode node = root; // Traverse the tree
while (node != null) { // Value of ancestor/parent node.
int parentVal = node.val; if (pVal > parentVal && qVal > parentVal) {
// If both p and q are greater than parent
node = node.right;
} else if (pVal < parentVal && qVal < parentVal) {
// If both p and q are lesser than parent
node = node.left;
} else {
// We have found the split point, i.e. the LCA node.
return node;
}
}
return null;
} @org.junit.Test
public void test() {
TreeNode tn11 = new TreeNode(6);
TreeNode tn21 = new TreeNode(2);
TreeNode tn22 = new TreeNode(8);
TreeNode tn31 = new TreeNode(0);
TreeNode tn32 = new TreeNode(4);
TreeNode tn33 = new TreeNode(7);
TreeNode tn34 = new TreeNode(9);
TreeNode tn43 = new TreeNode(3);
TreeNode tn44 = new TreeNode(5);
tn11.left = tn21;
tn11.right = tn22;
tn21.left = tn31;
tn21.right = tn32;
tn22.left = tn33;
tn22.right = tn34;
tn31.left = null;
tn31.right = null;
tn32.left = tn43;
tn32.right = tn44;
tn33.left = null;
tn33.right = null;
tn34.left = null;
tn34.right = null;
tn43.left = null;
tn44.right = null;
System.out.println(lowestCommonAncestor1(tn11, tn21, tn22).val);
System.out.println(lowestCommonAncestor1(tn11, tn21, tn32).val);
System.out.println(lowestCommonAncestor2(tn11, tn21, tn22).val);
System.out.println(lowestCommonAncestor2(tn11, tn21, tn32).val);
}
}