Minimum Depth of Binary Tree

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

solution：

采用BFS的方式遍历tree即可。 同时为了知道是在哪一层 加入一个每一层结束的signal node。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int minDepth(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(root == null) return 0;
        LinkedList<TreeNode> q = new LinkedList<TreeNode>();
        q.add(root);
        q.add(new TreeNode(-999));//一个singal，可以用 INT_MAX 代替比较好。
        int num = 0;
        while(q.size() != 0){
            TreeNode tn = q.remove();
            if(tn.val == - 999){
                num ++;
                q.add(new TreeNode(-999));
            } 
            else{
                if(tn.left != null) q.add(tn.left);
                if(tn.right != null) q.add(tn.right);
                if(tn.left == null && tn.right == null){
                    return num + 1;
                }
            }
            
        }
        return num + 1;//这句永远都跑不到
    }
}