Leetcode865

求最深的叶子节点的最近公共祖先

我的解法很懒，就是先找到最深的节点集合，然后一个个地merge

    int depth = -1;
    Map map = new HashMap<>();
    Map deMap = new HashMap<>();
    List list = new ArrayList<>();
    public TreeNode subtreeWithAllDeepest(TreeNode root) {
        dfs(root, null, 1);
        dfs(root, 1);

        TreeNode res = list.get(0);

        for (int i = 1; i< list.size(); i++)  {
            res = merge(list.get(i), res);
        }
        return res;

    }

    private TreeNode merge(TreeNode treeNode, TreeNode res) {
        while (deMap.get(treeNode) > deMap.get(res)) {
            treeNode = map.get(treeNode);
        }

        while (deMap.get(res) > deMap.get(treeNode)) {
            res = map.get(res);
        }

        while(treeNode.val != res.val) {
            treeNode = map.get(treeNode);
            res = map.get(res);
        }
        return res;
    }

    private void dfs(TreeNode root, int d) {
        if (root == null) return;
        if (d == depth) {
            list.add(root);
        }
        dfs(root.left, d + 1);
        dfs(root.right, d + 1);
    }

    private void dfs(TreeNode root, TreeNode parent, int d) {
        if (root == null) return;
        map.put(root, parent);
        deMap.put(root, d);
        depth = Math.max(depth, d);
        dfs(root.left, root, d + 1);
        dfs(root.right, root, d + 1);

    }

 

看了评论区，大神的解法

 

大致思路是，最深的叶子节点深度为0，它们的父节点深度1，爷爷节点为2。。。

如果左右子树深度相同，那么最深的叶子节点两边都有，如果不同，那么最深的叶子节点在比较大的那边。

https://leetcode.com/problems/smallest-subtree-with-all-the-deepest-nodes/discuss/146808/One-pass

下面的回复中比较详细的解析和比较清晰的注释和改造后的代码

    public TreeNode subtreeWithAllDeepest(TreeNode root) {
        return deep(root).getValue();
    }

    public Pair deep(TreeNode root) {
        if (root == null) return new Pair(0, null);
        Pair l = deep(root.left), r = deep(root.right);

        int d1 = l.getKey(), d2 = r.getKey();
        return new Pair(Math.max(d1, d2) + 1, d1 == d2 ? root : d1 > d2 ? l.getValue() : r.getValue());
    }