LeetCode(111)——Minimum Depth of Binary Tree

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.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its minimum depth = 2.

最小高度，有個限制要注意，如果一個節點的左子樹爲空時，不能進行min比較，要返回右子樹的高度+1，右子樹爲空同理。

AC:

public int minDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        
        int leftDepth = minDepth(root.left);
        
        int rightDepth = minDepth(root.right);
        
        if (leftDepth == 0) {
            return rightDepth + 1;
        }
        
        if (rightDepth == 0) {
            return leftDepth + 1;
        }
        
        
        return Math.min(leftDepth, rightDepth) + 1;
    }