Minimum Depth of Binary Tree

就recursive做，简单题

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

非recursive做法就是binary tree level order traversal的应用小拓展一下, 这种做法逻辑非常清晰，一般可以见雍正

    public int minDepth(TreeNode root) {
        if(root==null) return 0;
        int res=1, curNum=1,nextNum=0;
        LinkedList<TreeNode> q = new LinkedList<TreeNode>();
        q.add(root);
        while(!q.isEmpty()){
            curNum--;
            TreeNode n = q.poll();
            if(n.left==null && n.right==null) return res;
            if(n.left!=null){
                nextNum++;
                q.add(n.left);
            }
            if(n.right!=null){
                nextNum++;
                q.add(n.right);
            }
            if(curNum==0){
                res++;
                curNum=nextNum;
                nextNum=0;
            }
        }
        return res;
    }