algorithm-class/大厂刷题班/class45/Problem_2035_PartitionArrayIntoTwoArraysToMinimizeSumDifference.java

package class45;

import java.util.HashMap;
import java.util.TreeSet;

public class Problem_2035_PartitionArrayIntoTwoArraysToMinimizeSumDifference {

	public static int minimumDifference(int[] arr) {
		int size = arr.length;
		int half = size >> 1;
		HashMap<Integer, TreeSet<Integer>> lmap = new HashMap<>();
		process(arr, 0, half, 0, 0, lmap);
		HashMap<Integer, TreeSet<Integer>> rmap = new HashMap<>();
		process(arr, half, size, 0, 0, rmap);
		int sum = 0;
		for (int num : arr) {
			sum += num;
		}
		int ans = Integer.MAX_VALUE;
		for (int leftNum : lmap.keySet()) {
			for (int leftSum : lmap.get(leftNum)) {
				Integer rightSum = rmap.get(half - leftNum).floor((sum >> 1) - leftSum);
				if (rightSum != null) {
					int pickSum = leftSum + rightSum;
					int restSum = sum - pickSum;
					ans = Math.min(ans, restSum - pickSum);
				}
			}
		}
		return ans;
	}

	
	// arr -> 8   0 1 2 3      4 5 6 7
	// process(arr, 0, 4)  [0,4)
	// process(arr, 4, 8)  [4,8)
	// arr[index....end-1]这个范围上，去做选择
	// pick挑了几个数！
	// sum挑的这些数，累加和是多少！
	// map记录结果
	// HashMap<Integer, TreeSet<Integer>> map
	// key -> 挑了几个数，比如挑了3个数，但是形成累加和可能多个！
	// value -> 有序表，都记下来！
	// 整个过程，纯暴力！2^15 -> 3万多，纯暴力跑完，依然很快！
	public static void process(int[] arr, int index, int end, int pick, int sum,
			HashMap<Integer, TreeSet<Integer>> map) {
		if (index == end) {
			if (!map.containsKey(pick)) {
				map.put(pick, new TreeSet<>());
			}
			map.get(pick).add(sum);
		} else {
			process(arr, index + 1, end, pick, sum, map);
			process(arr, index + 1, end, pick + 1, sum + arr[index], map);
		}
	}

}