[leetcode] Minimum Depth of Binary Tree

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.

 https://oj.leetcode.com/problems/minimum-depth-of-binary-tree/

思路：递归实现，取左右节点中深度小的+1。

　　注意：如果左右子树其中一个为null，则应以不为null的子树深度为准，而不是以null的深度0为准。

public class Solution {

    public int minDepth(TreeNode root) {
        if (root == null)
            return 0;
        if (root.left == null)
            return minDepth(root.right) + 1;
        if (root.right == null)
            return minDepth(root.left) + 1;
        else

        {
            int minLeft = minDepth(root.left);
            int minRight = minDepth(root.right);
            return (minLeft < minRight ? minLeft : minRight) + 1;
        }
    }

    public static void main(String[] args) {
        TreeNode root = new TreeNode(10);
        root.left = new TreeNode(5);
        root.right = new TreeNode(5);
        root.right.left = new TreeNode(9);

        System.out.println(new Solution().minDepth(root));
    }
}

第二遍记录：注意各种终止情况的讨论。

public class Solution {

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

}

第三遍记录：

思路完全参考第二遍，写之前画图分析各种情况。