Count Univalue Subtrees(250/Medium)

Given a binary tree, count the number of uni-value subtrees.

A Uni-value subtree means all nodes of the subtree have the same value.

For example:
Given binary tree,

              5
             / \
            1   5
           / \   \
          5   5   5

return 4.

Java代码

private int count = 0;  

public int countUnivalSubtrees(TreeNode root) {  
    unival(root);  
    return count;  
}  

private boolean unival(TreeNode root) {  
    if(root == null)  
        return true;  

    if(root.left ==null && root.right == null) {  
        count++;  
        return true;  
    }  

    boolean left = unival(root.left);  
    boolean right = unival(root.right);  
    if(left && right && (root.left == null || root.left.val == root.val) && (root.right == null || root.right.val == root.val)) {  
            count++;  
            return true;  
    }  

    return false;  
}