236 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 thedefinition 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 allowa node to be a descendant of itself).”

        _______3______
       /              \
    ___5__          ___1__
   /      \        /      \
   6      _2       0       8
         /  \
         7   4

For example, the lowest common ancestor (LCA) of nodes5and1is3. Another example is LCA of nodes5and4is5, since a node can be a descendant of itself according to the LCA definition.

分治，左边找找，右边找找，两边都找到证明这个node是LCA。如果只有一边找到就上报，到上面再决定。

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    if (root == null || root == p || root == q) {
        return root;
    }

    TreeNode left = lowestCommonAncestor(root.left, p, q);
    TreeNode right = lowestCommonAncestor(root.right, p, q);

    if (left != null && right != null) {
        return root;
    } else if (left != null) {
        return left;
    } else if (right != null) {
        return right;
    } else {
        return null;
    }
}