Minimum Depth of Binary Tree，求树的最小深度

算法分析：递归和非递归两种方法。

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public class MinimumDepthofBinaryTree {           //递归，树的最小深度，就是它左右子树的最小深度的最小值+1     public int minDepth(TreeNode root) {         if(root == null)         {             return 0;         }         int lmin = minDepth(root.left);         int rmin = minDepth(root.right);         if(lmin == 0 && rmin == 0)         {             return 1;         }         if(lmin == 0)//左子树为空，右子树不为空，则最小深度为右子子树的最小深度+1         {             return rmin + 1;         }         if(rmin == 0)         {             return lmin + 1;         }         return Math.min(lmin, rmin)+1;     }                 //非递归，按层遍历，找到第一个叶子结点     public int minDepth2(TreeNode root) {         if(root == null)         {             return 0;         }         ArrayList<TreeNode> list = new ArrayList<>();         int count = 1;//初始值为1         list.add(root);         while(!list.isEmpty())         {             ArrayList<TreeNode> temp = new ArrayList<>();             for (TreeNode node : list) {                 if(node.left == null && node.right == null)//当节点左右子树都为空时，该节点为第一个叶子节点，该节点的深度即为树的最小深度                 {                     return count;                 }                 if(node.left != null)                 {                     temp.add(node.left);                 }                 if(node.right != null)                 {                     temp.add(node.right);                 }             }             count ++;//按层遍历，每循环一次，就是一层，层数加1             list = temp;         }         return count;     } }