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;
}