Lowest Common Ancestor of a Binary Tree -- LeetCode

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：如果left为空，则结果是right。若left为非空，而right为空，则结果在left。最后若两者皆不为空，则必定是p和q分别在两个子树里，不然不可能都返回了非NULL，此时直接返回root。

class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
if (!root || root == p || root == q) return root;
TreeNode *left = lowestCommonAncestor(root->left, p, q);
if (left && left != p && left != q) return left;//answer in left
TreeNode *right = lowestCommonAncestor(root->right, p, q);
if (!left) return right;//left is null, so answer depends on right
else if (!right) return left;//right is null, so answer in left
else return root;//both are not null, so root is the answer
}
};