给一棵二叉树和二叉树中的两个节点,找到这两个节点的最近公共祖先 LCA。两个节点的最近公共祖先,是指两个节点的所有父亲节点中(包括这两个节点),离这两个节点最近的公共的节点。每个节点除了左右儿子指针以外,还包含一个父亲指针parent,指向自己的父亲。
样例
对于下面的这棵二叉树
4
/ \
3 7
/ \
5 6
LCA(3, 5) = 4
LCA(5, 6) = 7
LCA(6, 7) = 7
代码
/**
* Definition of ParentTreeNode:
*
* class ParentTreeNode {
* public ParentTreeNode parent, left, right;
* }
*/
public class Solution {
/**
* @param root: The root of the tree
* @param A, B: Two node in the tree
* @return: The lowest common ancestor of A and B
*/
public ParentTreeNode lowestCommonAncestorII(ParentTreeNode root,
ParentTreeNode A,
ParentTreeNode B) {
// 用数组分别存储 A , B 结点向上寻找的结点
ArrayList<ParentTreeNode> pathA = getPath2Root(A);
ArrayList<ParentTreeNode> pathB = getPath2Root(B);
// 忘记 -1 会出现数组下标越界
int indexA = pathA.size() - 1;
int indexB = pathB.size() - 1;
ParentTreeNode lowestAncestor = null;
// 从最远的公共祖先开始寻找,找到最近的公共祖先
while (indexA >= 0 && indexB >= 0) {
// break 时不需要更新 lowestAncestor,
// 其上一个 pathA.get(indexA) == pathB.get(indexB) 的 lowestAncestor 是所需的
if (pathA.get(indexA) != pathB.get(indexB)) {
break;
}
lowestAncestor = pathA.get(indexA);
indexA--;
indexB--;
}
return lowestAncestor;
}
private ArrayList<ParentTreeNode> getPath2Root(ParentTreeNode node) {
ArrayList<ParentTreeNode> path = new ArrayList<>();
while (node != null) {
path.add(node);
node = node.parent;
}
return path;
}