LCA-二叉树公共祖先问题

1. 二叉搜索树
   leetcode 235. Lowest Common Ancestor of a Binary Search Tree

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root.val>p.val && root.val>q.val){
            return lowestCommonAncestor(root.left,p,q);
        }
        if(root.val<p.val && root.val<q.val){
            return lowestCommonAncestor(root.right,p,q);
        }
        return root;
    }
}

2. 普通二叉树
   leetcode 236. Lowest Common Ancestor of a Binary Tree

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root==null || root==p || root==q){
            return root;
        }
        TreeNode left = lowestCommonAncestor(root.left,p,q);
        TreeNode right = lowestCommonAncestor(root.right,p,q);
        if(left==null){
            return right;
        }
        if(right==null){
            return left;
        }
        return root;
    }
}

3. 带next指针指向父节点的LCA问题
   思路：得到p、q为头节点的两个链表，求链表的公共节点

Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
       TreeNode next; //指向父节点
 *     TreeNode(int x) { val = x; }
 * }

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode p, TreeNode q) {
        TreeNode phead = p;
        TreeNode qhead = q;
        int plength = 0;
        int qlength = 0;
        while(phead.next!=null){
            plength++;
            phead = phead.next;
        }
        while(qhead.next!=null){
            qlength++;
            qhead = qhead.next;
        }
        int len = plength - qlength;
        TreeNode fast = p;
        TreeNode slow = q;
        if(len<0){
            fast = q;
            slow = p;
        }
        for(int i=0; i<len; i++){
            fast = fast.next;
        }
        while(fast.next!=null && slow.next!=null && fast.val!=slow.val){
            fast = fast.next;
            slow = slow.next;
        }
        return fast;
    }
}