ac Lowest Common Ancestor of a Binary Tree

236-Lowest-Common-Ancestor-of-a-Binary-Tree/README.md

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+### 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 the [definition of LCA on Wikipedia](https://en.wikipedia.org/wiki/Lowest_common_ancestor): “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.
+
+### 方法（一）
+二种情况。
+1. v和w有直系关系，就像5和6或者6和3
+2. v和w没有直系关系
+第二种情况一定分别在T节点的左右子树上，而且也仅有T节点的左右子树分别有这两个节点。这里用递归找左右子树中是否有这两个节点。如果左右子树中都有，那祖先节点就是当前节点，如果只有左或者只有右子树上找到节点，说明是第一种情况返回找到的节点就可以了。
+

236-Lowest-Common-Ancestor-of-a-Binary-Tree/index.js

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+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @param {TreeNode} p
+ * @param {TreeNode} q
+ * @return {TreeNode}
+ */
+var lowestCommonAncestor = function(root, p, q) {
+    if (root == p || root == q || !root) {
+        return root
+    }
+    let left = lowestCommonAncestor(root.left, p, q)
+    let right = lowestCommonAncestor(root.right, p, q)
+
+    if (left && right) {
+        return root
+    } else if (left) {
+        return left
+    } else if (right) {
+        return right
+    } else {
+        return null
+    }
+};
+function TreeNode(val) {
+    this.val = val;
+    this.left = this.right = null;
+}
+const root = new TreeNode(4)
+root.left = new TreeNode(2)
+root.right = new TreeNode(6)
+root.left.left = new TreeNode(1)
+root.left.right = new TreeNode(3)
+root.right.left = new TreeNode(5)
+root.right.right = new TreeNode(7)
+
+console.log(lowestCommonAncestor(root, root.right.left, root.right))