Minimum Depth of Binary Tree

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.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int minDepth(TreeNode root) {
        if(null == root){//空树
            return 0;
        }
        else if(null != root && null == root.left && null == root.right)//只有根节点的树
            return 1;
        else if(null != root && null != root.left && null == root.right){//只有左子树
            return minDepth(root.left) + 1;
        }
        else if(null != root && null == root.left && null != root.right){//只有右子树
            return minDepth(root.right) + 1;
        }else{//左右子树都有
            int min = minDepth(root.left);
            if(min > minDepth(root.right))
                min = minDepth(root.right);
            return min + 1;
        }
    }
}

这道题主要是求根节点到叶子节点最短路径的长度，这里我借鉴了求树的深度算法。用的是递归实现的

如果是空树直接返回0

如果只有根节点返回1

如果只有左子树返回左子树的最短路径长度+1

如果只有右子树返回右子树的最短路径长度+1

如果有左右子树返回左右子树中最短路劲长度 + 1，这里需要比较左右子树最短路径长度，取最小那个返回