Leetcode: Median of Two Sorted Arrays
There are two sorted arrays A and B of size m and n respectively. Find the median of the two sorted arrays. The overall run time complexity should be O(log (m+n)).
2017/2/5更新:如果一定要每次扔一半,使得时间复杂度为O(log(m+n))。可以在第一个数组找前k1个,第二个数组找前k2个,使得k1+k2 == k, 分情况:
1. if A[k1] < B[k2], then A[k1]及之前的、B[k2+1]及之后的都不可能成为第k个元素,所以扔掉
2. else if A[k1] > B[k2], then A[k1+1]及之后的、 B[k2]及之前的都不可能成为第k个元素。扔掉
One e.g. to explain:
x: x1 x2 | x3 x4 x5 x6
y: y1 y2 y3 y4 y5 | y6 y7 y8
We are currently looking for 7th element, where we compare 2nd element in x and 5th element in y
Note that if x2 < y6 then x2 < all the elements on the right side. Similarily, if y5 < x3, y5 < all the elements on the right side
if we know x2 < y5, we know that x2 can't be the 7th. Because we can't find possible enough candidates to be less than x2 if x2 < x3 and x2 < y5
y6 can't be either, y6 > y5, ans so y6 > x2. There are already k elements in front of y6
1 public class Solution { 2 public double findMedianSortedArrays(int A[], int B[]) { 3 if((A.length+B.length)%2==1) 4 return helper(A,B,0,A.length-1,0,B.length-1,(A.length+B.length)/2+1); 5 else 6 return (helper(A,B,0,A.length-1,0,B.length-1,(A.length+B.length)/2) 7 +helper(A,B,0,A.length-1,0,B.length-1,(A.length+B.length)/2+1))/2.0; 8 } 9 10 private int helper(int A[], int B[], int i, int i2, int j, int j2, int k) 11 { 12 int m = i2-i+1; 13 int n = j2-j+1; 14 if(m>n) 15 return helper(B,A,j,j2,i,i2,k); 16 if(m==0) 17 return B[j+k-1]; //这是相对距离,不是相对于0,而是这轮起点j,所以决定了25行27行要减posA或posB 18 if(k==1) 19 return Math.min(A[i],B[j]); 20 int posA = Math.min(k/2,m); 21 int posB = Math.min(k-posA, n); 22 23 if(A[i+posA-1]<B[j+posB-1]) 24 return helper(A,B,i+posA,i2,j,j+posB-1,k-posA); 25 else 26 return helper(A,B,i,i+posA-1,j+posB,j2,k-posB); 27 } 28 }