牛客网2016.4.11（两个数相加为sum/计数一个int型的二进制有多少个1/二叉树是否左右对称）

求最小的两个数相加为sum

//求最小的两个数相加为sum
    public ArrayList<Integer> FindNumbersWithSum(int [] array,int sum) {
       List<Integer> list = new ArrayList<Integer>();
        if(array == null || array.length==0){
            return (ArrayList<Integer>) list;
        }

        for(int i=0;i<array.length-1;i++){
            for(int j=i+1;j<array.length;j++){
                if(array[i]+array[j] == sum){
                   list.add(array[i]);
                    list.add(array[j]);
                    break;
                }
            }
            if(list.size()!=0){
                break;
            }
        }
        //没找到就返回空集合
        return (ArrayList<Integer>) list;

    }

计数一个int型的二进制有多少个1，难点在负数需要补码存

    public int MoreThan0(int n) {
        int cnt = 0;
        char []c = Integer.toBinaryString(n).toCharArray();
        for (char d : c) {
            if(d=='1'){
                cnt++;
            }
        }
        return cnt;
    }

求一个二叉树是否左右对称

• 中心处理方法
  
  • 思路，利用栈队列的特性，进行层比较

import java.util.LinkedList;
import java.util.Stack;
public class Solution {
    boolean isSymmetrical(TreeNode pRoot) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        Stack<TreeNode> StackSon = new Stack<TreeNode>();
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        stack.push(pRoot);
        queue.offer(pRoot);
        TreeNode bt1 = null;
        TreeNode bt2 = null;
        while(stack.size()!=0 || queue.size()!=0){
            while(stack.size()!=0){
                //出队和出栈
                bt1 = stack.pop();
                bt2 = queue.poll();
                //比较镜像元素
                if(bt1==null || bt2==null){
                    break;
                }
                if(bt1.val != bt2.val){

                    return false;
                }
                //入队
                queue.offer(bt1.right);
                queue.offer(bt1.left);
                //栈的中转队列
                StackSon.push(bt2.right);
                StackSon.push(bt2.left);
            }

            while(StackSon.size()!=0){
                stack.push(StackSon.pop());
            }
        }
        return true;
    }
    public static void main(String[] args) {
        int a[] = {8,6,6,5,7,7,5};
        TreeNode root= new CreatTree().CreatTreeWay(a);
        System.out.println(new Solution().isSymmetrical(root));
    }
}

• 按层创建二叉树

import java.util.LinkedList;

public class CreatTree {
    TreeNode root = null;
    public TreeNode CreatTreeWay(int a[]){
        if(a==null || a.length==0 ){
            return null;
        }
        if(root == null){
            root = new TreeNode(a[0]);
        }else{
            return root;
        }
        LinkedList<TreeNode> list = new LinkedList<TreeNode>();
        list.offer(root);
        TreeNode node = null;
        for (int i = 1; i < a.length && list.size()!=0; i++) {
            node = list.poll();
            node.left = new TreeNode(a[i]);
            list.offer(node.left);
            if(i+1<a.length){
                node.right = new TreeNode(a[i+1]);
                list.offer(node.right);
            }
        }
        return root;
    }
    public static void main(String[] args) {
        int a[] = {8,6,6,5,7,7,5};
        new CreatTree().CreatTreeWay(a);
    }
}

• 忘了提供二叉树的结点类了

public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;
    }
}