671 Second Minimum Node In a Binary Tree

Given a non-empty special binary tree consisting of nodes with the non-negative value, where each node in this tree has exactlytwoorzerosub-node. If the node has two sub-nodes, then this node's value is the smaller value among its two sub-nodes.

Given such a binary tree, you need to output the second minimum value in the set made of all the nodes' value in the whole tree.

If no such second minimum value exists, output -1 instead.

Example 1:

Input: 
    2
   / \
  2   5
     / \
    5   7

Output: 5

Explanation: The smallest value is 2, the second smallest value is 5.

Example 2:

Input: 
    2
   / \
  2   2

Output: -1

Explanation: The smallest value is 2, but there isn't any second smallest value.

看到这题感觉想起了414 Third Maximum Number，就是套了个树的马甲。就最普通的遍历一遍，然后记录min1，min2的时候记得去重。

public int findSecondMinimumValue(TreeNode root) {
    if (root == null) {
        return 0;
    }

    int min1 = Integer.MAX_VALUE;
    int min2 = Integer.MAX_VALUE;

    Stack<TreeNode> stack = new Stack<>();                       
    stack.push(root);
    while (!stack.isEmpty()) {
        TreeNode cur = stack.pop();
        if (cur.val != min1 && cur.val != min2) {
            if (cur.val < min1) {
                min2 = min1;
                min1 = cur.val;
            } else if (cur.val < min2) {
                min2 = cur.val;
            }
        }

        if (cur.right != null) {
            stack.push(cur.right);
        }

        if (cur.left != null) {
            stack.push(cur.left);
        }
    }

    return min2 == Integer.MAX_VALUE ? -1 : min2;
}