(M)(Tree)Lowest Common Ancestor of a Binary Tree

来源：互联网发布：类似prezi的软件编辑：程序博客网时间：2024/05/21 21:34

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 {    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {        Map<TreeNode,TreeNode> map = new HashMap<TreeNode,TreeNode>();        map.put(root,null);        find(map,root,p);        find(map,root,q);                List<TreeNode> list1 = new ArrayList<TreeNode>();        list1.add(p);        List<TreeNode> list2 = new ArrayList<TreeNode>();        list2.add(q);        TreeNode parent1 = map.get(p);        while(parent1!=null)        {            list1.add(parent1);            parent1 = map.get(parent1);        }        TreeNode parent2 = map.get(q);        while(parent2!=null)        {            list2.add(parent2);            parent2 = map.get(parent2);        }                for(int i=0;i<=list1.size();i++)        {            if(list2.contains(list1.get(i)))                return list1.get(i);        }        return null;    }    public void find(Map<TreeNode,TreeNode> map,TreeNode root,TreeNode target)    {        if(root.val==target.val)            return;        if(root.left!=null)        {            map.put(root.left,root);            find(map,root.left,target);        }        if(root.right!=null)        {            map.put(root.right,root);            find(map,root.right,target);        }    }}

0 0