Leetcode 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 5and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

  

如果当前root 为null或者就是p q就return root

那么return的就是当前子树的祖先

如果说left和right都不为null说明pq分别在左右子树 此时return root

如果有一边为null return不为null的一边

全为null 直接return root

/** * 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) {    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 && right == null){       return root;   }else if(left != null && right != null){      return root;   } else{        return left == null ? right : left;   }}    }