Heapify

Given an integer array, heapify it into a min-heap array.

For a heap array A, A[0] is the root of heap, and for each A[i], A[i 2 + 1] is the left child of A[i] and A[i 2 + 2] is the right child of A[i].
Clarification
What is heap?

Heap is a data structure, which usually have three methods: push, pop and top. where "push" add a new element the heap, "pop" delete the minimum/maximum element in the heap, "top" return the minimum/maximum element.

What is heapify?
Convert an unordered integer array into a heap array. If it is min-heap, for each element A[i], we will get A[i 2 + 1] >= A[i] and A[i 2 + 2] >= A[i].

What if there is a lot of solutions?
Return any of them.
Example
Given [3,2,1,4,5], return [1,2,3,4,5] or any legal heap array.

public void heapify(int[] A) {
    if(A == null || A.length == 0) {
        return ;                
    }
    for(int i = A.length / 2; i >= 0; i--) {
        siftUp(A, i);
    }
}

public void siftUp(int[] A, int k) {
    while(k < A.length) {
        int min = k;
        if(2 * k + 1 < A.length && A[2 * k + 1] < A[min]) {
            min = 2 * k + 1;
        }

        if(2 * k + 2 < A.length && A[2 * k + 2] < A[min]) {
            min = 2 * k + 2;
        }
        if(k == min) {
            break;
        }
        int tmp = A[k];
        A[k] = A[min];
        A[min] = tmp;
        k = min;
    }
}