【LeetCode 0111】【递归/分治】二叉树的最小深度

111. 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.

Note: A leaf is a node with no children.

Example 1:

**Input:** root = [3,9,20,null,null,15,7]
**Output:** 2

Example 2:

**Input:** root = [2,null,3,null,4,null,5,null,6]
**Output:** 5

Constraints:

• The number of nodes in the tree is in the range [0, 10^5].
• -1000 <= Node.val <= 1000

Idea

分治递归：
分别算出左子树最小深度 和 右子树最小深度
返回 1+左右子树之间的最小深度

JavaScript Solution

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var minDepth = function(root) {
    if(!root){
        return 0
    }
    if(!root.left ){
        return minDepth(root.right) + 1
    }
    if( !root.right){
        return minDepth(root.left) + 1
    }
    return Math.min(minDepth(root.left),minDepth(root.right)) + 1
};