Solution2.java

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 *     // Constructor initializes an empty nested list.
 *     public NestedInteger();
 *
 *     // Constructor initializes a single integer.
 *     public NestedInteger(int value);
 *
 *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 *     public boolean isInteger();
 *
 *     // @return the single integer that this NestedInteger holds, if it holds a single integer
 *     // Return null if this NestedInteger holds a nested list
 *     public Integer getInteger();
 *
 *     // Set this NestedInteger to hold a single integer.
 *     public void setInteger(int value);
 *
 *     // Set this NestedInteger to hold a nested list and adds a nested integer to it.
 *     public void add(NestedInteger ni);
 *
 *     // @return the nested list that this NestedInteger holds, if it holds a nested list
 *     // Return empty list if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
class Solution {
    public NestedInteger deserialize(String s) {
        if (s.charAt(0) != '[') {
            return new NestedInteger(Integer.parseInt(s));
        }
        Deque<NestedInteger> stk = new ArrayDeque<>();
        int x = 0;
        boolean neg = false;
        for (int i = 0; i < s.length(); ++i) {
            char c = s.charAt(i);
            if (c == '-') {
                neg = true;
            } else if (Character.isDigit(c)) {
                x = x * 10 + c - '0';
            } else if (c == '[') {
                stk.push(new NestedInteger());
            } else if (c == ',' || c == ']') {
                if (Character.isDigit(s.charAt(i - 1))) {
                    if (neg) {
                        x = -x;
                    }
                    stk.peek().add(new NestedInteger(x));
                }
                x = 0;
                neg = false;
                if (c == ']' && stk.size() > 1) {
                    NestedInteger t = stk.pop();
                    stk.peek().add(t);
                }
            }
        }
        return stk.peek();
    }
}