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：递归post-order dfs来upwards累积
Time Complexity: O(N) Space Complexity: O(N) 递归缓存
Solution Code：
class Solution {
public int minDepth(TreeNode root) {
if(root == null) return 0;
int left = minDepth(root.left);
int right = minDepth(root.right);
return (left == 0 || right == 0) ? left + right + 1: Math.min(left,right) + 1;
}
}
