[Jvava] List的add底层实现原理_list保留数字add

list的add方法底层实现原理是基于指针，list对象指向添加元素的内存地址，对应内存地址元素发生改变，它的值也会发生改变。
在下面的代码中：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> pathSum(TreeNode root, int targetSum) {
        List<List<Integer>> res = new ArrayList();
        if( root == null ) return res;
        List<Integer> path = new ArrayList();
        getSum(root, targetSum, res, path, 0);
        return res;

    }

    public void getSum(TreeNode root, int targetSum, List<List<Integer>> res, List<Integer> path, int sum) {
        sum += root.val;
        path.add(root.val);
        if( (root.left == null) && (root.right == null) ) {

            if( sum == targetSum ) {

                // System.out.println(path.toString());
                // System.out.println(path.size());
                // List<Integer> tmp = new ArrayList();
                // for(Integer p:path) {
                //    tmp.add(p);
                // }
                res.add(path);

            } 
            return;
            // path.remove(path.size()-1);
        }
        if(root.left != null ) {
            getSum(root.left, targetSum, res, path, sum);
            path.remove(path.size()-1);
        }
        if(root.right != null ) {
            getSum(root.right, targetSum, res, path, sum);
            path.remove(path.size()-1);
        }
    }


}
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path发生改变，对应的res也会改变。
而下面的代码中，res会保留原本值，根本原因是，用了新的内存来保存相应的值。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> pathSum(TreeNode root, int targetSum) {
        List<List<Integer>> res = new ArrayList();
        if( root == null ) return res;
        List<Integer> path = new ArrayList();
        getSum(root, targetSum, res, path, 0);
        return res;

    }

    public void getSum(TreeNode root, int targetSum, List<List<Integer>> res, List<Integer> path, int sum) {
        sum += root.val;
        path.add(root.val);
        if( (root.left == null) && (root.right == null) ) {

            if( sum == targetSum ) {

                // System.out.println(path.toString());
                // System.out.println(path.size());
                List<Integer> tmp = new ArrayList();
                for(Integer p:path) {
                    tmp.add(p);
                }
                res.add(tmp);

            } 
            return;
            // path.remove(path.size()-1);
        }
        if(root.left != null ) {
            getSum(root.left, targetSum, res, path, sum);
            path.remove(path.size()-1);
        }
        if(root.right != null ) {
            getSum(root.right, targetSum, res, path, sum);
            path.remove(path.size()-1);
        }
    }


}
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