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

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深度理解：a,b的最低公共父节点为c,那么a,b一定是c的左右子树上面(先不考虑a==c||b==c)。 也就是说，a在c的左侧，b在c的右侧，不可能是c的同一侧，因为那个节点不是最低的，理解什么叫最低，所以ab一定分布在c的两侧。

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {        if(root==NULL||p==root||q==root) return root;        TreeNode*left=lowestCommonAncestor(root->left,p,q);        TreeNode*right=lowestCommonAncestor(root->right,p,q);        if(left!=NULL&&right!=NULL) return root;        return left==NULL?right:left;    }};