Minimum Depth of Binary Tree leetcode java

题目：

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.

 

题解：

递归解法急速判断左右两边子树哪个depth最小，要注意如果有个节点只有一边孩子时，不能返回0，要返回另外一半边的depth。

递归解法：

1      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     }

 非递归解法：

 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     }