/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Pair {
public boolean isUnival;
public int count;
public Pair(boolean isUnival, int count) {
this.isUnival = isUnival;
this.count = count;
}
}
class Solution {
public int countUnivalSubtrees(TreeNode root) {
return univalSubtreeCount(root).count;
}
private Pair univalSubtreeCount(TreeNode root) {
if (root == null) {
return new Pair(true, 0);
}
if (root.left == null && root.right == null) {
return new Pair(true, 1);
}
Pair left = univalSubtreeCount(root.left);
Pair right = univalSubtreeCount(root.right);
//an important step to check whether left and right subtrees are all valid
if (left.isUnival && right.isUnival) {
if (root.left == null && root.val == root.right.val) {
return new Pair(true, left.count + right.count + 1);
}
if (root.right == null && root.val == root.left.val) {
return new Pair(true, left.count + right.count + 1);
}
if (root.left != null && root.right != null && root.val == root.left.val && root.val == root.right.val) {
return new Pair(true, left.count + right.count + 1);
}
}
return new Pair(false, left.count + right.count);
}
}
For this problem, the most important thing is that we need both isValid boolean value and int count value, so we can either set a count[] array and pass in the helper function or set count to be global variable.
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
private int count = 0;
public int countUnivalSubtrees(TreeNode root) {
helper(root);
return count;
}
private boolean helper(TreeNode root) {
if (root == null) return true;
boolean left = helper(root.left);
boolean right = helper(root.right);
if (left && right) {
if (root.left != null && root.val != root.left.val) return false;
if (root.right != null && root.val != root.right.val) return false;
count++;
return true;
}
return false;
}
}
'Brute Force' verbose version:
For this problem, the most important thing is that we need both isValid boolean value and int count value, so we can either set a count[] array and pass in the helper function or set count to be global variable.