problem

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).”

Given the following binary tree: root = [3,5,1,6,2,0,8,null,null,7,4]

Example 1:

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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:

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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.

Note:

All of the nodes’ values will be unique. p and q are different and both values will exist in the binary tree.

approach 1:递归

算法

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class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null || p == root || q == root) {
            return root;
        }
        TreeNode leftResult = lowestCommonAncestor(root.left, p, q);
        TreeNode rightResult = lowestCommonAncestor(root.right, p, q);
        if (leftResult != null && rightResult != null) {
            return root;
        }
        if (leftResult == null) {
            return rightResult;
        }
        if (rightResult == null) {
            return leftResult;
        }
        return null;
    }
}

复杂度

• time:O(n)
• space:O(n)