236 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 p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Output: 3
Explanation: The LCA of nodes 5 and 1 is 3.

Example 2:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

Example 3:

Input: root = [1,2], p = 1, q = 2
Output: 1

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前序遍历，如果遇到目标节点，返回 true ，最小公共祖先的左右两个字节点的遍历都会返回 true 。边界条件其中一个节点是公共祖先，需要考虑到。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    TreeNode ret;
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root==null || p==null || q==null) return null;
        ret=null;
        find(root, p, q);
        return ret;
    }
    
    private boolean find(TreeNode root, TreeNode p, TreeNode q){
        if(root==null) return false;
        if(root==p || root==q) {
            if(ret==null && (find(root.left, p, q) || find(root.right, p, q))) ret=root;
            return true;
        }
        boolean left=false, right=false;
        left=find(root.left, p, q);
        right=find(root.right, p, q);
        if(left && right) ret=root;
        return (left || right);
    }
}