687 Longest Univalue Path

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int ans = 0;
    public int longestUnivaluePath(TreeNode root) {
        if (root == null) return 0;
        dfs(root);
        return ans;
    }

    private int dfs(TreeNode root){
        if (root == null) return 0;

        int leftAns = dfs(root.left);
        int rightAns = dfs(root.right);

        int goLeft = 0, goRight = 0;
        if (root.left != null && root.left.val == root.val) goLeft += leftAns + 1;
        if (root.right != null && root.right.val == root.val) goRight += rightAns + 1;
        ans = Math.max(ans, goLeft + goRight);

        //System.out.println("Node: " + root.val + " ans: " + ans + " left: " + goLeft + " right: " + goRight);

        return Math.max(goLeft, goRight);
    }
}

652 Find Duplicate Subtrees

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<TreeNode> ans;
    Map<String, Integer> hash;
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        hash = new HashMap<>();
        ans = new ArrayList<>();
        dfs(root);

        return ans;
    }

    private String dfs(TreeNode root){
        if (root == null) return "#";

        String cur = root.val + "," + dfs(root.left) + "," + dfs(root.right);

        if (!hash.containsKey(cur)){
            hash.put(cur, 1);
        }
        else{
            hash.put(cur, hash.get(cur) + 1);
        }

        if (hash.get(cur) == 2) ans.add(root);

        return cur;
    }
}

504 Base 7

class Solution {
    public String convertToBase7(int num) {
        if (num < 0) return "-" + convertToBase7(-num);
        if (num < 7) return num + "";
        return convertToBase7(num / 7) + num % 7;
    }
}