題目
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 v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”
_______6______
/ \
___2__ ___8__
/ \ / \
0 _4 7 9
/ \
3 5
For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.
答案
treat BST as a general tree
class Solution {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if(root == p || root == q || root == null) return root;
TreeNode left = lowestCommonAncestor(root.left, p, q);
TreeNode right = lowestCommonAncestor(root.right, p, q);
if(left == p && right == q)
return root;
if(right == p && left == q)
return root;
return (left != null)? left:right;
}
}
using BST property
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if(root==null) return null;
if(p.val<root.val&&q.val<root.val) {
return lowestCommonAncestor(root.left,p,q);
}
else if(p.val>root.val&&q.val>root.val){
return lowestCommonAncestor(root.right,p,q);
}
else
return root;
}
