Kth smallest sum form two sorted arrays

Given two integer arrays sorted in ascending order and an integer k. Define sum = a + b, where a is an element from the first array and b is an element from the second one. Find the kth smallest sum out of all possible sums. 

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int getKthSmallest(int a[],int b[],int k){ Objects.requireNonNull(a); Objects.requireNonNull(b); if(k<0||k>a.length || k>b.length)throw new RuntimeException("invalid input"); return Math.min(a[k-1]+b[0],b[k-1]+a[0]); }