【树】Lowest Common Ancestor of a Binary Tree（递归）

题目：

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：

节点a与节点b的公共祖先c一定满足：a与b分别出现在c的左右子树上；或者a,b一方是祖先元素。

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @param {TreeNode} p
 * @param {TreeNode} q
 * @return {TreeNode}
 */
var lowestCommonAncestor = function(root, p, q) {
    if(root==null){
        return;
    }
    if(root==p||root==q){
        return root;
    }

    var l=lowestCommonAncestor(root.left,p,q);
    var r=lowestCommonAncestor(root.right,p,q);

    if(l&&r){
        return root;
    }else{
        return l?l:r;
    }

};