Update 56. Merge Intervals.md

Author: wz30

56. Merge Intervals.md

@@ -46,6 +46,8 @@ class Solution {
 ```
 
 ## java solution with less coding
+- 关键是sort 整个array， 然后比较previous interval's end and current interval's start
+- 反证法 证明这种algo 总能得到最优解
 ```
 class Solution {
     public int[][] merge(int[][] intervals) {