[leedcode 236] Lowest Common Ancestor of a Binary Tree
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.
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { List<TreeNode> list1; List<TreeNode> list2; public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) { /* 前序遍历二叉树,查找p和q。 一旦找到某一节点,就将其返回,不再遍历其子树。 若某一子树遍历完后都没有找到p或q,则返回null。 每一次递归完成后,比较当前根节点的左右子树是否找到p或q。 若左右子树都找到了(p和q分别在左子树和右子树中),返回当前根节点。 若只有一个子树找到了,另一子树返回为空(p和q都在同一子树中),返回不为空的子树节点(不一定是当前根节点的子树根节点)。 若两个子树都没找到(p和q都不在该子树中),返回左子树(实则返回null)。*/ //注意此时lowestCommonAncestor函数返回值意义已经变了,代表从root起点,是否存在两个节点(之一即可) /*if(root==null) return null; if(p==root||q==root) return root; TreeNode left=lowestCommonAncestor(root.left,p,q); TreeNode right=lowestCommonAncestor(root.right,p,q); if(left!=null&&right!=null){ return root; } return left!=null?left:right;*/ //第二种方法:使用两个链表保存从根节点到指定节点的路径中遍历到的值,然后转变为求两个链表第一个不同的节点 list1=new ArrayList<TreeNode>(); list2=new ArrayList<TreeNode>(); getPath(root,p,list1); getPath(root,q,list2); int i=0; int j=0; for(;i<list1.size()&&j<list2.size();i++,j++){ if(list1.get(i)!=list2.get(j)) break; } return list1.get(i-1); } public boolean getPath(TreeNode root,TreeNode p,List<TreeNode> list){ if(root==null) return false; list.add(root); if(root==p) return true; boolean isExist=false; isExist=getPath(root.left,p,list); if(!isExist){ isExist=getPath(root.right,p,list); } if(!isExist){ list.remove(list.size()-1); return false; }else return true; } }