Missing Ranges 解答

Question

Given a sorted integer array where the range of elements are in the inclusive range [lower, upper], return its missing ranges.

For example, given [0, 1, 3, 50, 75], lower = 0 and upper = 99, return ["2", "4->49", "51->74", "76->99"].

Answer

遍历数组，如果arr[i] > arr[i-1] + 1则说明arr[i]与arr[i-1]中有missing range。注意考虑开头和结尾。

public class Solution {
    public List<String> findMissingRanges(int[] nums, int lower, int upper) {
        List<String> result = new ArrayList<>();
        if (nums == null || nums.length == 0) {
            addRange(result, lower, upper);
            return result;
        }
        int len = nums.length;
        // process head
        if (nums[0] > lower) {
            addRange(result, lower, nums[0] - 1);
        }
        for (int i = 1; i < len; i++) {
            if (nums[i] > nums[i - 1] + 1) {
                addRange(result, nums[i - 1] + 1, nums[i] - 1);
            }
        }
        // process tail
        if (nums[len - 1] < upper) {
            addRange(result, nums[len - 1] + 1, upper);
        }
        return result;
    }
    
    private void addRange(List<String> result, int low, int high) {
        StringBuilder sb = new StringBuilder();
        sb.append(low);
        if (high > low) {
            sb.append("->");
            sb.append(high);
        }
        result.add(sb.toString());
    }
}