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.

方法：遞迴，自底向上。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
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 minDepth(root.right) + 1;
        if (root.right == null) return minDepth(root.left) + 1;
        return Math.min(minDepth(root.left), minDepth(root.right)) + 1;
    }
}
 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int minDepth(TreeNode root) {
        if (root == null) return 0;
        Stack<Input> inputs = new Stack<>();
        Stack<Integer> outputs = new Stack<>();
        inputs.push(new Input(root, 0));
        while (!inputs.isEmpty()) {
            Input input = inputs.pop();
            if (input.state == 0) {
                if (input.root == null) {
                    outputs.push(0);
                    continue;
                } else if (input.root.left == null && input.root.right == null) {
                    outputs.push(1);
                    continue;
                } else if (input.root.left == null) {
                    input.state = 1;
                    inputs.push(input);
                    inputs.push(new Input(input.root.right, 0));
                    continue;
                } else if (input.root.right == null) {
                    input.state = 1;
                    inputs.push(input);
                    inputs.push(new Input(input.root.left, 0));
                    continue;
                } else {
                    input.state = 2;
                    inputs.push(input);
                    inputs.push(new Input(input.root.left, 0));
                    continue;
                }
            } else if (input.state == 1) {
                Integer output = outputs.pop();
                output ++;
                outputs.push(output);
                continue;
            } else if (input.state == 2) {
                input.value = outputs.pop();
                input.state = 3;
                inputs.push(input);
                inputs.push(new Input(input.root.right, 0));
                continue;
            } else if (input.state == 3) {
                Integer right = outputs.pop();
                Integer output = Math.min(input.value, right) + 1;
                outputs.push(output);
                continue;
            }
        }
        return outputs.pop();
    }
}
class Input {
    TreeNode root;
    int state;
    Input(TreeNode root, int state) {
        this.root = root;
        this.state = state;
    }
    int value;
}