Question Definition
Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.
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).”
_______3______
/ \
___5__ ___1__
/ \ / \
6 _2 0 8
/ \
7 4
For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.
Java Solution
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
List<TreeNode> listp = new LinkedList<>();
List<TreeNode> listq = new LinkedList<>();
if(!findNode(root, p, listp))
return null;
if(!findNode(root, q, listq))
return null;
TreeNode result = null;
int i = 0;
for(; i < listp.size() && i < listq.size() && listp.get(i) == listq.get(i); i++);
if(i < listp.size())
result = listp.get(i - 1);
else if(i < listq.size())
result = listq.get(i - 1);
return result;
}
private boolean findNode(TreeNode root, TreeNode p, List<TreeNode> list){
if(root == p){
list.add(p);
return true;
}
if(root == null)
return false;
list.add(root);
if(findNode(root.left, p, list))
return true;
if(findNode(root.right, p, list))
return true;
list.remove(list.size() - 1);
return false;
}
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