LeetCode:Lowest Common Ancestor of a Binary Search Tree

Lowest Common Ancestor of a Binary Search Tree

Total Accepted: 37087 Total
Submissions: 97884 Difficulty: Easy

Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.
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).”

_______6______
       /              \
    ___2__          ___8__
   /      \        /      \
   0      _4       7       9
         /  \
         3   5

For example, the lowest common ancestor (LCA) of nodes

2

and

8

is

6

.
Another example is LCA of nodes

2

and

4

is

2

,
since a node can be a descendant of itself according to the LCA definition.

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(M) Lowest Common Ancestor of a Binary Tree

code:

/**
 * 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(p == root || q == root) return root; // 有结点是root，返回结果
        int flag1 = p->val < root->val ? 1:0;
        int flag2 = q->val > root->val ? 1:0;
        if(!(flag1 ^ flag2)) // (1,1)(0,0)如果p、q一个在左、一个在右，root即为最小公共祖先
            return root;
        else if(flag1) // (1,0) p、q都在左边，遍历左子树
            return lowestCommonAncestor(root->left, p, q);
        else  // (0,1) p、q都在右边，遍历右子树
            return lowestCommonAncestor(root->right, p, q);
    }
};