Minimum Depth of Binary Tree leetcode java

題目：

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.

 

題解：

遞迴解法急速判斷左右兩邊子樹哪個depth最小，要注意如果有個節點只有一邊孩子時，不能返回0，要返回另外一半邊的depth。

遞迴解法：

     public int minDepth(TreeNode root) {
2         if(root == null)
3             return 0;
4         int minleft = minDepth(root.left);
5         int minright = minDepth(root.right);
6         if(minleft==0 || minright==0)
7             return minleft>=minright?minleft+1:minright+1;
8         return Math.min(minleft,minright)+1;
9     }

 非遞迴解法：

 1     public int minDepth(TreeNode root) {
 2         if(root == null)
 3             return 0;
 4         
 5         int depth = 1;//The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.
 6         LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
 7         queue.add(root);
 8         int curnum = 1;
 9         int nextnum = 0;
10         while(!queue.isEmpty()){
11             TreeNode cur = queue.poll();
12             curnum--;
13             
14             if(cur.left == null && cur.right == null)
15                 return depth;
16             
17             if(cur.left != null){
18                queue.add(cur.left);
19                nextnum++;
20             }
21             
22             if(cur.right != null){
23                 queue.add(cur.right);
24                 nextnum++;
25             }
26             
27             if(curnum == 0){
28                 curnum = nextnum;
29                 nextnum = 0;
30                 depth++;
31             }
32         }
33         return depth;
34     }