236. Lowest Common Ancestor of a Binary Tree

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本文介绍了一种寻找二叉树中两节点的最低公共祖先(LCA)的有效算法。通过分别获取两节点到根节点的路径，找出路径上最近的共同节点即为所求。示例包括了具体的代码实现。

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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.

思路:这题有上题不同，这是一颗普通的二叉树，我们可以用分别得到到p，q的两个路径，看两个路径的最接近的就是最低公共祖先

代码如下(已通过leetcode)

public class Solution {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
List<TreeNode> list1=new ArrayList<TreeNode>();
List<TreeNode> list2=new ArrayList<TreeNode>();
getpath(root,p,list1);
getpath(root,q,list2);

for(int i=0;i<=list1.size();i++) {
if(list2.contains(list1.get(i))) return list1.get(i);
}
return null;
}

private boolean getpath(TreeNode root, TreeNode p, List<TreeNode> list) {
// TODO Auto-generated method stub
if(root==null) return false;
if(root==p) {
list.add(root);
return true;
}else {
if(getpath(root.left, p, list)||getpath(root.right, p, list)) {
list.add(root);
return true;
}
return false;
}

}

}