327. Count of Range Sum

327. Count of Range Sum

題目連接：https://leetcode.com/problems...

這題實際就是給定範圍內的range sum，divide and conquer的方法。一路計算prefixSum[0:i]，並把結果放進tree裏面，而後計算到prefixSum[0:j+1]的時候，找tree裏面有沒有知足條件的prefixSum[0:i]，這裏的條件是lower <= sum[0:j+1] - sum[0:i] <= upper，那麼可知sum[0:j+1] - upper <= sum[0:i] <= sum[0:j+1] - lower，那麼這個就一個recursion就行了。注意一開始把0加進去，考慮結果是sum[0:j]的狀況，還有要用long型，以避免sum會overflow

public class Solution {
    public int countRangeSum(int[] nums, int lower, int upper) {
        int n = nums.length;
        if(n == 0) return 0;
        // binary search tree
        Node root = new Node(0);
        int res = 0;
        long prefixSum = 0;
        for(int i = 0; i < n; i++) {
            prefixSum += nums[i];
            res += findNumInBound(root, lower, upper, prefixSum);
            insert(root, prefixSum);
        }
        return res;
    }
    
    private int findNumInBound(Node node, long low, long up, long sum) {
        // base case
        if(node == null) return 0;
        // range: sum - up <= node.val <= sum - low
        if(node.val < sum - up) return findNumInBound(node.right, low, up, sum);
        else if(node.val > sum - low) return findNumInBound(node.left, low, up, sum);
        else return 1 + findNumInBound(node.left, low, up, sum) + findNumInBound(node.right, low, up, sum);
    }
    
    private void insert(Node node, long value) {
        while(node != null) {
            if(node.val > value) {
                if(node.left == null) {
                    node.left = new Node(value);
                    break;
                }
                node = node.left;
            }
            else {
                if(node.right == null) {
                    node.right = new Node(value);
                    break;
                }
                node = node.right;
            }
        }
    }
    
    class Node {
        long val;
        Node left;
        Node right;
        Node(long val) { this.val = val; }
    }
}

仍是能夠binary index tree來作，要統計sum[0:j+1] - upper <= sum[0:i] <= sum[0:j+1] - lower範圍內的個數，就是用sum。參考博客：
http://bookshadow.com/weblog/...

public class Solution {
    public int countRangeSum(int[] nums, int lower, int upper) {
        int n = nums.length;
        if(n == 0) return 0;
        // prefix array
        long[] prefixSum = new long[n];
        for(int i = 0; i < n; i++) {
            prefixSum[i] = (i > 0 ? prefixSum[i-1] : 0) + nums[i];
        }
        long[] sorted = Arrays.copyOf(prefixSum, prefixSum.length);
        Arrays.sort(sorted);
        // binary index tree
        map = new HashMap();
        int idx = 1;
        for(long sum : sorted) {
            if(!map.containsKey(sum)) map.put(sum, idx++);
        }
        // build tree
        BIT t = new BIT(idx);
        int res = 0;
        for(int i = 0; i < n; i++) {
            int l = binarySearch(sorted, prefixSum[i] - upper - 1);
            int r = binarySearch(sorted, prefixSum[i] - lower);
            res += t.sum(r) - t.sum(l);
            if(prefixSum[i] >= lower && prefixSum[i] <= upper) res += 1;
            t.add(map.get(prefixSum[i]), 1);
        }
        return res;
    }
    Map<Long, Integer> map;
    // find the last element <= val
    private int binarySearch(long[] arr, long val) {
        int l = 0, r = arr.length - 1;
        while(l < r) {
            int mid = l + (r - l) / 2 + 1;
            if(arr[mid] <= val) l = mid;
            else r = mid - 1;
        }
        if(arr[l] > val) return 0;
        return map.get(arr[l]);
    }
    
    class BIT {
        int n;
        int[] tree;
        BIT(int n) { this.n = n; tree = new int[n]; }
        
        protected int sum(int i) {
            int res = 0;
            while(i > 0) {
                res += tree[i];
                i -= i & -i;
            }
            return res;
        }
        
        protected void add(int i, int val) {
            while(i < n) {
                tree[i] += val;
                i += i & -i;
            }
        }
    }
}