# 1123. Lowest Common Ancestor of Deepest Leaves
###### tags: `Leetcode` `Medium` `LCA` `DFS`
Link: https://leetcode.com/problems/lowest-common-ancestor-of-deepest-leaves/
## 思路
### 思路一 自己的思路 O(N) O(N)
和下面两题思路很像，建议复习的时候把所有LCA的题目一起看
[0236. Lowest Common Ancestor of a Binary Tree](https://hackmd.io/8Sj0WCYbRFadYo4hDTXcyg)
[1676. Lowest Common Ancestor of a Binary Tree IV](https://hackmd.io/rfFxWl_vSpmNuuDBZnDChQ)

先通过bfs找到最深的所有node里面最左边和最右边的一个，然后找他们的LCA
### 思路二 大神的思路 O(N) O(N) (学习思路)
具体看代码
**比较重要的一点是ta是将递归(line 15-16)前和递归后分开考虑的**
前面由于是从上往下 所以用来计算最深的深度
当执行到16行后面的时候 deepest已经是最深深度了，所以从下往上找LCA

## Code
### 思路一

class Solution {
    public TreeNode lcaDeepestLeaves(TreeNode root) {
        if(root == null) return null;
        TreeNode leftMost = null, rightMost = null;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while(!q.isEmpty()){
            int size = q.size();
            for(int i = 0;i < size;i++){
                TreeNode curr = q.poll();
                if(i==0) leftMost = curr;
                if(i==size-1) rightMost = curr;
                if(curr.left!=null) q.add(curr.left);
                if(curr.right!=null) q.add(curr.right);
            }
        }
        return findLCA(root, leftMost, rightMost);
    }
    public TreeNode findLCA(TreeNode root, TreeNode p, TreeNode q){
        if(root==null | root==p | root==q) return root;
        TreeNode left = findLCA(root.left, p, q);
        TreeNode right = findLCA(root.right, p, q);
        if(left!=null && right!=null) return root;
        if(left!=null) return left;
        else return right;
    }
}

### 思路二

class Solution {
    int deepest;
    TreeNode lca;
    public TreeNode lcaDeepestLeaves(TreeNode root) {
        deepest = 0;
        dfs(root, 0);
        return lca;
    }
    public int dfs(TreeNode root, int deep){
        //记录最深的level是第几层
        deepest = Math.max(deepest, deep);
        if(root == null) return deep;
        
        //recurse function, 先到最left， 再到最right，同时把deepest更新成最下面的值(见上面代码)， 然后从下面往上走，如果遇见一个node左右高度相等且到达的最深的level是deepest（见下面代码），就把lca设置成node
        int leftDeepest = dfs(root.left, deep+1);
        int rightDeepest = dfs(root.right, deep+1);
        
        if(leftDeepest==deepest && rightDeepest==deepest){
            lca = root;
        }
        return Math.max(leftDeepest, rightDeepest);
    }
}